Check the Cards that come with the Great Old Ones. They will have summoning requirements, among other things. Standard practice is to make Great Old Ones available equal to number of players -1 (ie: 3 Great Old Ones for a 4-player game). A player may summon one anytime they meet the summoning requirements.
exactly, on a first come first served base. Most of them have requirements like "have your great old one at a controlled gate and pay 8 power" or something similar. the IndyGOOs are a lot of fun to play with! they can dramatically change your strategy!