I thought this was interesting, it comes from the Dave Kalama Blog,

"The ten man rule is really a windsurfing rule, if there are ten or more surfers on a break you can’t windsurf there. It only applies to Ho’okipa. It makes good sense for windsurfing–you can’t see surfers well when you’re ripping on a windsurfer, and if a break is crowded it’s likely that someone is going to get run over, so the lifeguards recommended a ten man rule and it became a regulation–ten surfers in a break and the windsurfers can’t use it.

A bunch of long-time Maui SUP surfers got together and recommended that SUP surfers use the same rule at Ho’okipa because it relieved the tension between surfers and sweepers. It’s not a regulation for SUP, just a recommendation, and we sweepers at Ho’okipa have pretty much adopted it. You rarely see a Stand Up Paddler at Ho’okipa these days, but I think it’s more because we’ve figured out that there are so many fine spots to use that it just doesn’t make sense to crowd into places that already have too many people."