Do lots and lots of research, there is more to it than just putting two fish together, it is fun and I recommend that you spawn Betta splendens at least once......but be ready and don't expect to make any money.....lol.....
Most female can have up to 300+ eggs, but with the bad, weak, and unfertile eggs and some that the female eats during spawning....sometimes you can get a nest of 100-150 eggs and from that as long as the male is not an egg eater...you can expect maybe 60-100 to reach wiggler stage and then 10-60 free swimming stage and then the numbers will either stabilize..... but most first time hobbyist end up with a manageable number between 5-30+...which in my opinion is a easy number to work with to start until you get the hang of it and get all the stuff you need to house the males once they need separated and time to care for them...for best growth and development they need lots of live food and lots of water changes....or you end up with ratty, stunted looking bettas with shortened swim bladders....
But it is fun, the research is fun, lots of spawning methods to pick from too, its just finding which one works best for you and the breeders.
I like to have at least 3 live food cultures for the fry and at least 2 or more live foods for the breeder to condition them with before spawning ever takes place, if the breeders are not in tip top shape it can affect the quality of eggs and offspring IME
What spawning method are you going to try and besides the BBS what other food do you plan to feed?