Depends on what your end goal is and how much free time you have. It also depends on how fast you can sell. I would suggest getting through a breeding season before you buy too many more animals. Eventually you reach a point that managing the animals becomes work. I know my absolute limit is around 100 holdbacks as long as I have a full time job. I have 50 right now. I will probably produce around 100 babies this season. A couple of those clutches are being bred specifically for future projects. I will probably end up keeping around twenty from what I am making this year depending on the number of females that pop out. So you can see how the collection can grow very quickly without buying anything.
I would suggest that you narrow your scope on what you are attempting to create also. I am working almost the entire BEL complex, albino, lavender albino, yellow belly, gravel, spark, fire, pinstripe, spider, enchi, pastel and some mystery gene I have running around in my collection that I don't have a handle on. I am working too many genes in my opinion as a hobbyist. My advice in hindsight would be find the best of what you can afford and concentrate on producing quality animals.