user6969: I'm making a project and I'm using multiple 1 digit 7 segment displays. I haven't plugged them all on my arduino because currently I'm just testing. The problem is on the I/O ports. Obviously I need to use shift registers and since it's my first time ever using them I need to know more about them. After googling, I found that the specific model I bought outputs 8 pins and you can stack up to 8 in series. So lets do the math. 8 shift register x 8 pins each SR = 64 new pins It seems cool but assuming I'm using 28 displays let's do the math again.
28 displays, each display has 8 pins (8 for segments and 2 as common cathode)
28 x 8 = 224 required pins.
Is is possible to expand the I/O pins by 224 pins? How many shift registers can I stack together? Also which is better, the single digit display or the 4 digit display?

user10794: You can buy these displays on eBay for a few dollars. A bit of soldering and you are done (although also pre-assembled ones are available). They can be daisy-chained. Example:

You only need 5 wires to the Arduino: Gnd/5V/MOSI/SCK/SCK.

Also, 64-dot displays for displaying arbitrary text:

Again, only 5 wires.
I have Arduino libraries for driving both sorts of display on 
Interfacing LED displays with the MAX7219 driver .

user2881: You can chain as many shift registers as you want.
Most commonly, these displays are multiplexed. Only one digit is lit at a time, cycling though all the digits fast enough, so the eye can't tell. This is however not very good practice for 28 digits, as the brightness will become 1/28th (though you could create groups of digits, and multiplex those groups separately).
There are specialized chips that do this multiplexing for you. E.g. the Max7219. This chip can multiplex 8 digits, with 8 segments. So you'd need 4 of these to light up 28 digits.

6969: <p>I'm making a project and I'm using multiple 1 digit 7 segment displays. I haven't plugged them all on my arduino because currently I'm just testing. The problem is on the I/O ports. Obviously I need to use shift registers and since it's my first time ever using them I need to know more about them. After googling, I found that the specific model I bought outputs 8 pins and you can stack up to 8 in series. So lets do the math. <code>8 shift register</code> x <code>8 pins each SR</code> = <code>64 new pins</code> It seems cool but assuming I'm using 28 displays let's do the math again.
28 displays, each display has 8 pins (8 for segments and 2 as common cathode)
<code>28</code> x <code>8</code> = <code>224</code> required pins.
Is is possible to expand the I/O pins by 224 pins? How many shift registers can I stack together? Also which is better, the single digit display or the 4 digit display?</p>
 10794: <p>You can buy these displays on eBay for a few dollars. A bit of soldering and you are done (although also pre-assembled ones are available). They can be daisy-chained. Example:</p>

<p><img src="https://i.stack.imgur.com/dEbzN.jpg" alt="6 x 4-digit 7-segment display modules"></p>

<p>You only need 5 wires to the Arduino: Gnd/5V/MOSI/SCK/SCK.</p>

<p><img src="https://i.stack.imgur.com/YhNOe.jpg" alt="modules in operation"></p>

<p>Also, 64-dot displays for displaying arbitrary text:</p>

<p><img src="https://i.stack.imgur.com/8Z5oV.jpg" alt="8x8 LED matrix"></p>

<p>Again, only 5 wires.</p>

<p>I have Arduino libraries for driving both sorts of display on <a href="http://www.gammon.com.au/forum/?id=11516" rel="nofollow noreferrer">
Interfacing LED displays with the MAX7219 driver </a>.</p>
 2881: <p>You can chain as many shift registers as you want.</p>

<p>Most commonly, these displays are multiplexed. Only one digit is lit at a time, cycling though all the digits fast enough, so the eye can't tell. This is however not very good practice for 28 digits, as the brightness will become 1/28th (though you could create groups of digits, and multiplex those groups separately).</p>

<p>There are specialized chips that do this multiplexing for you. E.g. the Max7219. This chip can multiplex 8 digits, with 8 segments. So you'd need 4 of these to light up 28 digits.</p>
