Q:

Raspberry pi and mq series sensors

I want to make an air quality monitoring system for a University using the Raspberry Pi 3 model B.
1) How many inputs does the Raspberry Pi have and can they also be used as outputs?
2) What is the area covered by MQ-series of sensors. 

A:

There are 26 usable GPIO (general purpose input-output) pins on a Raspberry Pi (2,3,A+,B+,Zero). They are usable as either inputs or outputs, depending on how you configure them.
I believe the MQ series sensors work by air naturally moving into the sensor, being heated and then getting analysed by the sensor inside the casing. So, a very limited area in answer to your second question. They're normally good for what you want though.
You will require an analog-to-digital converter to read the values from the MQ series of sensors as they are analog sensors. I recommend the MCP3008 as it gives you 8 analog inputs. You can use a simple Python programming library called GPIO Zero to read the values (http://raspi.tv/2016/using-mcp3008-to-measure-temperature-with-gpio-zero-and-raspio-pro-hat) or do it the old fashioned way with RPi.GPIO and another library. It's shown in the pic below how to wire it all up. You don't need to use the prototyping board, you can use a breadboard if you like. It's just what came to mind first. The ProHAT, if you do want one to play with, is available here.

MCP3008 used to measure temperature on a Raspberry Pi prototyping board, the RasPiO ProHAT

