well if you ask this question in the "feeders" forum you will probably get lots more answers.
however, when starting with dubias, it all depends on how you go about things. If you want to start a colony and be able to feed off of it while it is growing then obviously you are going to want a mix of adults and nymphs. I would suggest getting as many adults as possible especially females. I would have 1 adult male for every 3-5 adult females, put them in a rubbermaid tub with some egg crates, food, water crystals and heat source and just leave them alone for a few months other than to give more food, water, etc..
then in a few months you will have lots of babies and it should be ready to start feeding from.