question about breeding

I know that its best to wait till they are 16 yr. old to breed, and the reasoning behind limiting breeding but...

1. do you breed the same pair more than once, and if so why?

2. do you breed the same horse to different horses, then what do you do with the offspring, if it is ones you want keep not outside breeding's? 

I'm thinking I have one more question but I cant think of what it was.

I personally only breed once and at age 20 - I won't go any younger unless there's an odd circumstance like I need to find a partner for a horse that is slightly older than the others. I go primarily for stat increase, hence waiting until 20 . . . if I'm working with colour on a breed I like to take my chance with genetics for the foal I end up with, so I don't bother breeding a pair multiple times.