How many different sensors can I use on my Arduino?

Is it possible to have several types of gas sensors (Methane/smoke and CO2) and both WiFi and GSM shields, and temperature, humidity and pressure sensors on one board?

I'm currently using Arduino Mega with WiFi shield and one sensor for both temperature and humidity!

Yes. Obviously you want to choose the correct board, preferably one with enough pins for everything you want to connect.

Steve

Bluelight82: I'm currently using Arduino Mega with WiFi shield and one sensor for both temperature and humidity!

I imagine there are quite a few vacant pins then, so that ought to tell you something.

if you use I2C, up to 127 sensors and displays combined, but you won't get continuous sensing and display updating if you use a huge number of I2C addresses. if you use SPI, an unknown but high number. 3 pins for SPI bus, one CS pin per sensor. if you use SPI for an SD card, the sensors and the SD card will have to contend for use of the bus. if you use Serial, you get 4 hardware serial ports and a whole pot of software serial ports, and if you are a clever dog, you can use multiplexers to increase the number of sensors, but again, you won't get continuous sensing and display updating

it depends on how many pins you use for output, and how many analog pins you need, how often you need to update the display, how much data you need to log, et cetera.

I would recommend using a spreadsheet to keep track of what you have available versus what you plan to use.