What kind of dats first off? Some get larger than others. In general though, they grow pretty slow. By pretty slow I dont mean "sort of slow, 1" a month" I mean a snails pace. Especially at first because theyll be shy for a while and not want to eat. I cant speak for everyone but anytime Ive bought datnoids with the exception of 2, its taken 2 to 4 weeks for them to start eating shrimp and another 2 months or so for them to actually come out and be somewhat comfortable with me. That being said, all the ones I buy are over 5" and babies may be easier to train. Id say get a solid handful, maybe 7 to 12 and sell them off as they grow until youre down to around 4 or 5. Eventually youll need something bigger but your tank would be fine for at least 4 or 5 years easy. So youll probably upgrade before they outgrow it.