Hi there! It depends on the age of your chams and how quickly you think they will go through them. I don’t have a Dubia colony but I do have a colony of red runners. For one Cham it’s too many. As far as quantity that’s up to you. There are ways to keep the population under control if you need to.
4 Dubia per cham every feeding. 8 Dubia(total) per feeding @ 15 feedings a month = 120 Dubia a month is your minimum replacement rate. 120 a month replacement rate is nothing for a healthy colony. Im feeding two adults as well, when I began my colony I also bought a few hundred to feed off while my colony developed. It timed out perfectly, my colony hit full replacement rate when I was about 3/4 of the way through the Dubia I bought for feeding off.