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.
Have fun, love them, and keep asking questions!