gsm shield and com ports

hellow there,

i am kinda newbie in this world of arduino so don't get mad if am asking such basic stuffs. i have a project to deal with, the project will need arduino to cummunicate with computer program to be developed using serial communication as well as the arduino will be hooked up with gsm shiled for sms notification.

My question is how many pins does gsm shield need on the arduino, second is that will it be possible for gms shield as well as the computer (for serial communication) to connect in the ardunio all together as my friend told me its not possible because they both use serial communication but he is not sure. On top of that is that my project will require 10 digital pins and 1 interupt pins, i was thinking if mega is going to be the best option.

It depends on which shield, but typically they use two for serial communication and one for software power control.