Hi Billy!
Male rats at the appropriate age can mate around 400 times in one days time. A successful ejaculation by the male can be confirmed by him cleaning his penis immediately after coupling with the female. Otherwise it's just foreplay or "Mock Mating".
one male, or your 'Stud rat', is suitable for breeding innumerable females, I have successfully produced 8 pregnant females within an hour from the same male on one occasion.
The easiest way to find a rat suitable for being your stud is to look for basic desirable features (as a feederbreeder that means to you that it looks healthy with no obvious genetic defects) with the size and general shape of a brick hahaha.
Males around a year old are my preferred age for studs, younger ones do not have the influx of hormones making them less aggressive breeders.
-Rat Pimp