Since you do not know the genetics of these bettas, you will have absolutely no clue what the fry will look like.
The amount that survive will greatly vary, you wont know until it happens.
Veiltails are very hard to find homes for, so keep that in mind in case you end up with a couple hundred fish.
Please read the stickys and do a lot of research before you breed. It is more complicated than most people think.
If you have any more questions, I suggest you ask in the breeding section to learn from experienced breeders.