Hi – I am not an equine reproductive specialist, but here is some info from someone who is:

From http://articles.extension.org/pages/29348/stallion-breeding-management :

Semen collection schedules

Collecting or breeding once per week decreases the weekly sperm output compared with breeding or collecting daily or every other day. Sperm output per week is similar whether collecting every other day or daily. Therefore, for maximum amounts of sperm collection and minimum labor, stallions should be collected every other day. The every-other-day schedule is typical for farms using artificial insemination.

Managers breeding stallions by natural cover may need them to cover mares more frequently than every other day. More frequent collection intervals will not result in a significant increase in weekly sperm output. Rather, individual collections will have fewer spermatozoa. Whether or not these decreases are significant enough to lessen fertility when breeding every day or twice daily will depend on the stallion’s spermatozoa production capability.

The stallion’s sex drive and his ability to produce spermatozoa will place limits on how often he can successfully breed mares. Sex drive, as monitored by reaction time and number of mounts necessary before ejaculation, usually will not be affected by increasing the breeding frequency from every other day to daily collections. However, managers must treat stallions as individuals and regulate collection intervals appropriately. Overuse can cause decreases in sex drive, observed by the stallion’s disinterest in breeding, longer mounting times required before breeding, or lack of ejaculation during breeding.

Also from this article: The number of mares that can be booked successfully to a stallion depends on the stallion’s spermatozoa output, the stallion’s sex drive, and the number of times each mare must be bred. A common booking limit is 30 to 40 mares per season with natural mating. This number can be larger if the manager utilizes palpation and ultrasound to accurately determine ovulation time in mares. [Note: some stallions are booked for up to 200 breedings in a season.]

Practical advice: if mares’ ovaries are palpated so mating can be timed properly, stallions won’t need to breed each mare multiple times and will therefore be able to breed more mares. One mare a day seems like a reasonable goal. Some stallions will be able to successfully cover and settle more mares than this, others fewer. Large testicle are indicators of higher sperm production capacity, as well.