
A fit male should be able to mate every day or even more often.
In nature a dog and bitch would mate as often as they wished over about a week (from the LH surge at earliest, about 2 days before ovulation, and to when the ova die off about 4 days after ovulation).
The only reason tom restrict mating that I can think of in the situation where dog and bitch are happy to do so, is where the dog is needed to service another bitch within a short space of time, so time would be needed to replenish reserves.
http://www.labbies.com/reproduction4.htm".......Since frequent ejaculation (daily collection of semen for 5 to 7 days) can cause a reduction in sperm output, studs in high demand may experience less than optimal fertility at certain times throughout their reproductive years. For this reason, it is recommended that sperm from valuable studs be collected and cryopreserved in sufficient quantities early in the male's career to ensure future availability. To this end, collection of semen on an "every other day" basis typically allows time for replenishment of sperm reserves............
........The amount of reserved sperm will depend on the frequency and interludes between ejaculations. Total sperm counts in a sexually rested male encompass sperm reserves plus daily sperm produced by the testes. Sperm reserves are reportedly depleted by once per day ejaculation for 5 to 7 days."
Of course multiple matings are not necessary to achieve pregnancy if timing is good, but multiple matings do generally improve chances of conception and litter size, and generally supervised properly will do no harm, and are probably better than having a male witter after the bitch by having restricted access.
When I have had stud dogs to stay, I have only separated them from the bitch when I have been out and at night, and even then the bitch was crated in same room as the male. Mot have chosen to mate the bitch after morning ablutions and then just be part of the gang of girls.
A male I bred came to do all his stud work at my home where his chosen ladies came to stay too. He was first used at 6 years and mated 6 bitches and all became pregnant.