Beer leagues around here allow 4 per game and then equalizer after that. When I fill in for those games I never really have to worry about hitting the limit because I rarely bat more than 4 times and nobody else can hit to save their lives.
Upper division USSSA leagues have a straight equalizer rule, as do most tournaments. All HRs are walked off to save time.
I've been in tournaments all over the West, and the HR rules are crazy sometimes. Some allow zero HRs. Some allow 2 for the team besides one guy designated as the unlimited HR hitter. Sometimes they make a HR result in an out and clears the bases...or ends the inning. One tournament made a HR result in an out, ended the inning, and then that spot in the lineup was an auto out for the rest of the game...super lame. If our sponsors weren't footing the bill i wouldn't have even showed up for that.
The most fun I've had playing softball was an open tourney with unlimited HRs. There were people lined up behind the OF wall tossing the bombs back in the moment they landed, and it was really fun watching everyone tee off over and over.