In an orchestral situation, it is a terrible idea to rely on the conductor - they may not be able to cue you or instead are going to give a more important cue.
Just because you are not playing does not mean you're allowed to sit and wait for a bus.
Scores that have been edited well will have "cues" written in the parts, so, say you zone out accidentally or lose your place after 200 measures
, you see the cue for an oboe solo in your part
and you know where you are.
Also, here are a couple good habits to get into: raise a finger or flick your leg each time you get to a rehearsal mark - this is a great way to keep a section together (trombones in Mahler, for example.)
when counting measures, give one number to the entire measure, and take the measure's duration to say the number to yourself.
Ex: Ooonnnnneeeee Twwwooooooo Threeeeee etc etc.
Counting this way greatly reduces counting confusion.
Like others have mentioned - listen to others around you; know what they are doing.
Just like in a play, it's good to know everyone's lines around your lines so that if someone else screws up you're still good to go.
music.stackexchange.com/users/3518/jjmusicnotes
Adding to Dom's answer, the key is often in listening for cues in other people.
Find out what other parts are doing around the time you come back in and before, so you can listen for that sign that you're about to come back in.
music.stackexchange.com/users/6629/Alexander Troup
In an orchestra, let the conductor do the work for you
- they should let you know your moment is coming.
In a smaller group, say a string quartet, you wouldn't expect such long rests (after all, you're 25% of the ensemble) -
but learn what the other parts sound like and use that to time your return.
So instead of "Now I count 64 bars", it's "I'll come back in after the third repetition of the main melody".
music.stackexchange.com/users/1252/slim
You don't actually have to count every bar that you don't play especially if you go many bars without playing anything.
Most large form pieces and even Jazz pieces are broken down into smaller parts i.e. an A section, a B section, C section, ect.
So you could have sections A, B, and C be 32 bars each
and you many not start playing until 4 bars into section C and instead of just counting 68 bars
you could listen for the start of the C section and then count 4 bars.
If you know the song inside and out you may not even need to count the 4 bars you may just be able to "feel" when you come in.
Also the conductor in an orchestra will cue the instruments coming in as it is what they do.
music.stackexchange.com/users/7222/Dom
I count bars 1-2-3-4 2-2-3-4 3-2-3-4 etc.
in my head and simultaneously keep track on my fingers.
Each finger denotes 1, thumb denotes 5, so I can count up to nine on one hand.
So with two hands I can keep count up to 99.
Major benefit of this is that I can multi-task - I keep counting whilst picking up mutes, pointing at music and so on.
After a while you can preset a count, so if the conductor says start at bar 8
you put down thumb and three fingers and start counting from there.
One advantage of designating fingers as 1 and thumb as 5 over binary is that it's much easier to preload a count on your fingers.
The disadvantage is that you can only count up to 100 on two hands, rather than 1000...
So say the conductor says "we're going from bar 67"
and you have an entry at bar 80,
you just preload 67 onto your fingers and start counting bars.
Compare this with having to preload 67 in binary onto your fingers.
music.stackexchange.com/users/9426/Brian THOMAS
Back in my orchestra days I played oboe, so I often had long rests.
Until you know when to come in based on what's happening in the other parts, as others have already said, you need to count bars of rest.
I used to count on my fingers, but in binary, from left thumb to right thumb.
That way, you can count to 2^10 or 1024, which is probably enough for most pieces.
It starts like this:(all fingers are in except those noted)1-
left thumb;2- left
index;3-
left thumb and index;4- left middle;5-
left thumb and middle;6- left index and middle;7- left thumb, index, and middle;8- left ring; etc.
It's fun and easy to learn.
music.stackexchange.com/users/26744/Scott Wallace
I also recommend counting in the way Brian THOMAS has explained (1-2-3-4-2-2-3-4).
However as mentioned this gets really complicated with 20+ bars rest.
Now from my experience with sheet music in orchestras you often already have long rests seperated whenever the key, general theme or athmosphere changes or new instrumental groups are introduced.
Try to pay attention to such separations and listen to the music.
Try to listen to the piece and figure out what happens at those seperations and use them as markers.
Generally listening to the piece you play a couple of times is a good idea.
I've had some long rests in certain pieces that were really hard to count but after listening to the music for a while I memorized my entranced by heart.
If your sheet doesn't have any seperations during long rests (or has seperations that don't make any sense) then make them yourself.
I usually look at the sheets of other instruments that I can visually see or hear well and when they suddenly start playing
I mark their entrance in my rest.
Solos also work great for this.
What also helps is looking for fermatas.
These are often really easy to hear and are a great means of orientation.
music.stackexchange.com/users/36217/Carpid
As a trombonist, I often had VERY long sections of rest.
Obviously I'm not going to sit there counting "one-two-three-four, two-two-three-four.....six-hundred-and-seventy-three-two-three-four...
The composer or copyist must supply me with landmarks, including a prominent cue played by another instrument a few bars before my entry.
Previous players of the part might leave me further pencilled-in information like "time for two pints here!".
music.stackexchange.com/users/17003/Laurence Payne
Playing in a jazz band there are often 5,6 or 8 bars of silence. This is no problem as I can count them on my fingers. But how do I count larger rests, for example 52, 112 or even longer rests, as in classical pieces?  What techniques do you have to count a large numbers of silent bars?
How do you count many bars of silence (rests)?
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<rhythm><performing><time-signatures><band><orchestra>
music.stackexchange.com/questions/15591
music.stackexchange.com/users/9440/WeSee
When your instrument has a long rest, it is good to find out what other parts are doing around the time you come back in and before, so you can listen for that sign that you're about to come back in. If you know the song inside and out you may not even need to count the 4 bars you may just be able to "feel" when you come in. In an orchestra, let the conductor do the work for you. You don't actually have to count every bar that you don't play especially if you go many bars without playing anything. The answer to this question will be subjective, but you may not need to count all the bars that you don't play. If you know the piece or other people's parts well enough, you may be able to simply recognise when you're supposed to start playing again. Alternatively, you could count the bars on your fingers or simply wait for the conductor to indicate when you should start again.
Knowing other people's lines is a good way to know when you should come in rather than counting rests. You could rely on the conductor to let you know when to come in. Some people would argue that it's not necessary to count every bar that you don't play. Some people count the bars on their fingers.