I've had my swordtails now for a year. The ones in the main community tank almost certainly had fry but such fry certainly got eaten by my golden barbs (which are voracious eaters- huge appetites & very quick swimmers!), so i never had any swordtail fry until 6months ago i put 3 females in a barely stocked planted tank (apart from an assortment of corys, a common plec & some clown plecos they are the only fish in the tank).
The females turned out to be pregnant and over the last 5months have been giving birth to fry in very small batchs. The tank has a lot of floating plants but i literally only find a few fry at a time. But even after 6months with no male swordtails about the 3 females are STILL pregnant and still giving birth to fry in small batches. I now have a total of 18 swordtail fry ranging from 5months old to 1month old (its pretty cool though as not one single fry as ever died, so all fry are originals
!). The largest fry is now visibly male & growing a sword on his tail (i call him a fry but he's pretty much big enough now to swim with the big fish now).
So i guess this is how it is with swordtails.
I kept guppys (& platys) for years on end but whenever they gave birth to fry (especially the guppys) it was like "BAM! 30 FRY IN THE TANK!"- i'd literally turn on the tank lights in the morning some days and there would be loads of guppy fry swimming everywhere hiding in the floating plants (BTW having floating plants in the tank, even fake plastic ones, will seriously increase the survival rates of the fry over the first few days) and the poor old guppy mothers would look noticeably slimmer & knackered. But with the female swordtails, it seems that while they can hold large amounts of fry like guppy females can, they prefer to give birth in small batches, using up sperm supplies they have stored from previous matings to create more fry in the absence of males (so at least with my female swordtails they always look like they are in some state of pregnancy).
These are just my personal experiences of keeping swordtails
.