Two mares are used for each stallion undergoing CEM protocols. Each mare is covered one time per stallion test. Then undergoes a series of several swab/test/cultures for a 3 week period. Mares obviously cannot be used for more than one stallion at a time.
Live cover breeding is not without risks, of course, and the impact on the mare of repeated invasive culture/swabbing protocols are worth considering. Outside of the TB race industry, most high end mares (and their owners) are used to AI only breeding protocols now and live cover is considered archaic. The truth lies somewhere in between, of course. TB mares undergo live cover at least once per season. Both live cover and AI mares that are bred undergo many invasive procedures themselves that we never question at all now. Mares being bred undergo pre-season BSE cultures, then repeated and sometimes daily pre-insemination rectal palpations/rectal ultrasounds, then AI procedure itself, and then more rectal ultrasounds to confirm pregnancy. If she does not become pregnant, mares undergo the same for another cycle or two or even 3 until the owner quits for the season. If pregnant, then more palpations/ultrasounds are carried out at certain points throughout gestation until she foals out. All privately owned mares undergo these things for at least one cycle and one gestation for every planned breeding. Then are subject to live foaling risks. Even ET donor mares that don’t carry foals beyond a few days are subjected multiple similar invasive processes.
Breeding mares for foals this way sure doesn’t sound awesome on paper, either. This is something worth thinking about, I believe, before judging CEM test mare life too harshly.