I'm looking for a rule, if there is one, for not allowing a pitcher to take warmup pitches (20-25) from the pitchers mound before start of game. This is an advantage as other teams' pitcher is only allowed a few pitches at start game. I can't find a rule mentioning this pre-game issue. Any info is appreciated.

The SSUSA Rulebook does not provide for any specific number of allowed pre-game warm-up pitches, nor does it prohibit or limit them. The only provision in the SSUSA Rulebook regarding warm-up pitches is reproduced below. We are not concerned about a potential disparity of pre-game pitches a pitcher may be able to take. Your pitcher may, of course, find a suitable 50' area on adjacent ground to throw as many as he would like, too. We don't see a competitive imbalance present in this.
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6.12 • WARM-UP PITCHES
At the beginning of each half-inning, or when a pitcher relieves another, not more than one minute may be used to deliver not more than three warm-up pitches. Play shall be suspended during this time. For excessive warm-up pitches, a pitcher shall be penalized by awarding a ball to the batter for each pitch. This does not apply if the umpire delays the start of play due to substitution, conference, injuries, etc. NOTE: A pitcher returning to pitch in the same half inning will not receive warm-up pitches.

Hi IcanRun - in addition to the advice from Senior Softball, please keep in mind the Umpire normally controls this. Most rec and all qualifiers have time limits. My goal is to see what I can do to keep the flow of the game going. I want both teams to be able to play as many innings as possible. If I allow excessive pitches, it just slows the game down.