Well, if you want the colony to grow quickly, buy at least 80 females and 15-20 males. You will eventually have lots of extras, but if you sell them off then the colony makes you even more money in addition to saving you money from having to buy less crickets. If you don't mind the wait, you can start out with 100 mixed and a few adults with a 3-6:1 female:male ratio.
You can use them completely instead of crickets, but remember, variety is a good thing. I feed my chameleons dubias but I also buy crickets to feed as well. Though right now, my male veiled has decided he will only eat one cricket a day or a few once a day if I wait a day or two between cricket feeding attempts. My female still prefers crickets or silkworms though.