I have bred my guppies and this is what I found.
When their belly starts to look a little more square, they are close. Also, once the fry are born, I put them either in a small breeder or net breeder. I didn't do this at first, just put them in a 10 gallon tank to grow out, but have more success when I keep them in their own tank for 3 days in a breeder. It is probably because they are weak at first and it is easier that way for them to get their food. Once I notice they are swimming to the top for their food without any trouble and buzzing around, I move them to a grow out tank. This has decreased the death rate of my fry substantially.
My guppies tend to have 1 to 4 fry at a time. I keep a very good eye out and have all the plants they need, but they usually only pop out a few at a time. I have had one that popped out over a dozen on one day, but only that one time.
