The numbers depend on a lot of factors, some of which you can control others of which you can't. You could get from a few to several hundred free swimming fry. These first numbers won't go up but can go down, So care must be taken. There are two good articles about breeding bettas, in that section of the page. Water conditions must be very good, and food should be fed at least 3 times a day if not as many as 5 times a day, a little at a time.
Live foods are the best.