CRASH COURSE IN HORSIE REPRODUCTION:

Gestation is 11 months long, so mares can have one foal a year, which for racehorses (since they have a universal January 1st birthday in the northern hemisphere) is usually from January through May.  Since horses can be expected to live to 20yo and beyond, and many mares start producing around age four or five, a mare can potentially have around 15 foals in her lifetime.  However, few mares can churn out foals year after year.  A vet once declared horses as the most difficult of domestic animals to get pregnant, with only a 60% success rate.  Most professional breeding farms can manage 80%+ conception rates.  Between conception problems, foalings problems (which might make it necessary to give the mare a year off), finances (such as the owner not wanting to pay a stud fee or otherwise not bother), or a mare no longer being bred due to poor quality foals, the average mare in the racing industry only has around a half dozen foals in her lifetime.