Breeding..?

If I wanted each pair to have 3 foals at what age would I breed them? The last day of 18,19,20? 

I'm not sure if horse can breed at 21 or not. 

Would 3 foals be a good number or is that too many? Most of my horses are paired up and would have 2-3 foals each. I do however have some stallions pair with 2 mares and I would like to breed the stud 4 times total and the mares each 2 times. Is that considering too many foals per stallion? 

Just want peoples opinions on breeding their horses. 

Yes, the recommended times to breed your horses for the best stats is on the last day of ages 18, 19, and 20. Horses cannot breed at age 21 because that is the age where their retirement begins. 
3 foals seems to be the typical foal count serious EV breeders tend to do. I personally do not feel like 4 foals per stallion is too much, I sometimes do 4 foals with my favorite Akhal-Tekes stallions.

Breeding ages for my horses is 18-20, but I do not breed them for my own until age 19. 
Basically they are put up for public stud/brood at age 18 and then I breed them at age 19 and 20 for 2 foals. I choose the foal with the best conformation out of the two and sell the other one.

Thanks, I won't be offering my horses for public breeding but I do plan to sell the majority of foals and focus more on showing in the future than breeding. Having to keep up with it all makes the game less fun for me. 

Thank you though the information you gave me helped :)