A young beardie eats tons, mine was eating 100 dubia a day!!! I dont like wasting money on adults because in a month or 2 your have adults anyways and you should let all colonies go for atleast 2-3 months before you start feeding them off. 25 adult females will give you about 500 roaches a month.
I would start with the largest colony you can afford at the time. May I ask where you are buying them from?