user-1: I'm currently looking to buy an Arduino, but never touched robotics before. Would anyone be able to give me an idea how many pins a sensor such a magnetometer would use? This is for Arduino Uno.
Sorry if this is stupid, just really have no idea how to approach Arduino design without having one here and can't get one for a few weeks at the least.
Thanks!

user31106: One, maybe two. It's impossible to say for sure without a concrete example. 
I would recommend looking up the type of sensor(s) you want to use, and read the data sheet(s). Things you'll want to consider are things like operating voltage (3.3v or 5v), current draw (>40mA?), are any additional passive components required (capacitors, resistors, ...), etc. 
Data sheets will describe in great detail, how to use a specific device.

user29436: 
Would anyone be able to give me an idea how many pins a sensor such a
  magnetometer would use?

Here below image provides the connection and pin requirements for magnetometer...

According to image magnetometer will require 4 pins of Arduino 2 pins of supply i.e +3 volt and Ground and other two pins will be any two Analog pins of Arduino.
And to connect other sensors with Arduino, you can find datasheet of desired sensor and can find how many pins that sensor is going to use.

user4143: 
How many pins are needed for a sensor?

As many pins as are needed for that sensor.
Every sensor is different. The datasheet or manual for the sensor or the chip or other device that the sensor is based around will tell you how the interface to it works - what each pin does, how you use it, etc.
It is up to you to read that documentation and learn how the sensor works to then be able to plan how best to connect it to your Arduino.
Some sensors require specific connections, such as the two I2C pins in your example sensor. Others may need the SPI pins. Then there are sensors that use analog inputs. That may be one pin for, say, an LDR, or three pins in the case of a 3-axis accelerometer. 
In short: RTFM.

user29796: Below image is the answer to your question.

According to this image, magnetometer required four pins of arduino.
GND of magnetometer ----> Arduino GND
Vcc of magnetometer ----> Arduino 3.3 Volt
SDA of magnetometer ----> Arduino Analog Pin 4
SCL of magnetometer ----> Arduino Analog Pin 5
I hope it will help you. 

-1: <p>I'm currently looking to buy an Arduino, but never touched robotics before. Would anyone be able to give me an idea how many pins a sensor such a magnetometer would use? This is for Arduino Uno.</p>

<p>Sorry if this is stupid, just really have no idea how to approach Arduino design without having one here and can't get one for a few weeks at the least.</p>

<p>Thanks!</p>
 31106: <p>One, maybe two. It's impossible to say for sure without a concrete example. </p>

<p>I would recommend looking up the type of sensor(s) you want to use, and read the data sheet(s). Things you'll want to consider are things like operating voltage (3.3v or 5v), current draw (>40mA?), are any additional passive components required (capacitors, resistors, ...), etc. </p>

<p>Data sheets will describe in great detail, how to use a specific device.</p>
 29436: <blockquote>
  <p>Would anyone be able to give me an idea how many pins a sensor such a
  magnetometer would use?</p>
</blockquote>

<p>Here below image provides the connection and pin requirements for magnetometer...</p>

<p><a href="https://i.stack.imgur.com/hSmNu.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/hSmNu.jpg" alt="Magnetometer"></a></p>

<p>According to image magnetometer will require 4 pins of Arduino 2 pins of supply i.e +3 volt and Ground and other two pins will be any two Analog pins of Arduino.</p>

<p>And to connect other sensors with Arduino, you can find datasheet of desired sensor and can find how many pins that sensor is going to use.</p>
 4143: <blockquote>
  <p>How many pins are needed for a sensor?</p>
</blockquote>

<p>As many pins as are needed for that sensor.</p>

<p><em>Every</em> sensor is different. The datasheet or manual for the sensor or the chip or other device that the sensor is based around will tell you how the interface to it works - what each pin does, how you use it, etc.</p>

<p>It is up to <em>you</em> to read that documentation and learn how the sensor works to then be able to plan how best to connect it to your Arduino.</p>

<p>Some sensors require specific connections, such as the two I2C pins in your example sensor. Others may need the SPI pins. Then there are sensors that use analog inputs. That may be one pin for, say, an LDR, or three pins in the case of a 3-axis accelerometer. </p>

<p>In short: RTFM.</p>
 29796: <p>Below image is the answer to your question.</p>

<p><a href="https://i.stack.imgur.com/COHxR.png" rel="noreferrer"><img src="https://i.stack.imgur.com/COHxR.png" alt="Connection"></a></p>

<p>According to this image, magnetometer required four pins of arduino.</p>

<p>GND of magnetometer ----> Arduino GND</p>

<p>Vcc of magnetometer ----> Arduino 3.3 Volt</p>

<p>SDA of magnetometer ----> Arduino Analog Pin 4</p>

<p>SCL of magnetometer ----> Arduino Analog Pin 5</p>

<p>I hope it will help you. </p>
