GSM modules

Most GSM modules use a serial interface to communicate with its host.

Can only guess which ones you are talking about.

The Raspi has only one serial port. So the total number out of the box is one.

If you use USB to serial adapters, you can up that number to as many as you would like.

This can give you four USB ports:

https://www.amazon.com/MakerSpot-4-Port-Stackable-Raspberry-Supply/dp/B01K9IVUYM

Load the USB serial port driver, config the Raspi to know which USB port is which.

Should work.

Good Luck, Have Fun, Learn Something NEW