It should be unlimited.
first of all, not one person on here has said how you would handle the lopsided games with subbing logically. Because of region games, there will always be lopsided scores. When you are a better team and you play teams like LR, OW, Rock Hill, Marlboro Co., Hartsville, etc., it allows some of your bench players to get considerable minutes. Your starters still get their run, but then some of the kids who dont play much, if any, in the big games get a chance to show what they can do. If you limit subs then you limit some of these players minutes.
Secondly, why is unlimited substituting an advantage? I think leaving players in the match is better than having these line changes, which I have seen very little of. the good teams tend to get in a flow and want very little changes to stay in it.
Thirdly, by limiting subs, will that really make everyone match fit? Or will it just slow down the pace of the game? Teams will just sit in more and counter even more than they already do if they are not fit.
Lastly, high level club players, ie premier and challenge level, are not asked to play 90 mins or sit, so why should a high school player who may only play during the high school season be asked to do something different? Lets be real, only at the big schools are they getting 18-20 soccer players if they are lucky. Do you think Heise is getting that many soccer players? some years he might, others he definitely is not.