It is Sunnah to recite Tasbeeh (Subhaan Rabbi al azim) at least three times while prostrating in prayer, the middle level is to recite it five times and the highest level is to recite it seven times.  However, reciting the Tasbih less than three times will not fulfill the Sunnah, rather the prayer will be Makruh.

Any amount of reading more than three is permissible and due to excess, there is reward , however, it should end with an odd number (3, 5, 7, 9, etc.).

There is an order for the imam to reduce the duties and offer a light prayer, so the imam should respect the worshipers and not recite such long tasbihs in the bowing and prostration that it burdens the worshipers and makes them angry.

The jurists have written that it is better for the imam to recite tasbihat five times while bowing and prostrating; So that the devotees get the opportunity to read the Tasbih at least three times with satisfaction.