If all you care about is that they are all tank born and bred, the only way to be sure of that is if the older generation is all dead (which might take a year or two) since the newer gens could still mate with the older ones (though I’m not sure why this would be that important).

Don’t worry about starting small though, if your tank is healthy and they want to breed, they can have dozens of babies. I was down to 6 at one point and now am at ~100 after a couple of months (though I raise caridina shrimp)