Is it possible to keep only female Platies in order to avoid reproduction?
I think of purchasing some Platies, but I read that they reproduce very frequently!
this is indeed the case, you can expect a litter from each female every 4 - 6 weeks.
Is it also possible to keep only females, and how many at least?
This is generally indeed possible, but it is usually not successful considering reproduction prevention: Platies store semen, therefore a female can give birth several times even if she has mated only once. This is virtually always the case with the fish sold in the trade, as you can avoid this only by separating the fry according to their genders before they become sexually mature. And the efforts are in vain if one single male was among the females...
An alternative is to add fish that eat the fry but do not molest the adult Platies. Most characin species are, for example, suitable in this case, so are dwarf cichlids - those, however, may possibly multiply rapidly instead!
Dr. Bodo Schnell