New to the forum so have no one vouching for me, but have lots of breeding experience. For those smaller species it should not be more than about twice a year to prevent egg-binding and exhaustion (larger birds only one). If you strive to have the maximum of two clutches per year it is important to provide a breeder diet that includes extra calcium and calories from sources such as breeder pellets, fresh cooked egg or powdered egg food, and a good seed mix once per day (sprouted if you can swing it). Spray millet is great for breeders but I recommend giving that particular favorite only in measured quantites each day so they still eat the other vital nutritional sources mentioned.
Oh also, you may find that some of those smaller species will seem to "want to breed" more than recommended. It is important to stop them from doing so because they will NOT always know to stop breeding when they've done too much. You will need to remove the nest box and reduce their daylight hours to no more than 12 hours per day in order to take them out of breeding mode in many cases. Please do this between clutches so that your hens can have the longest and happiest breeding life possible! I move my breeders to my non-breeding room which has fewer daylight hours when they're done. Also remember that there are age limits to breeding which vary among species and can be Googled.