What Lynne says is true, but what you can do is create a new user group and just increase the number of PMs allowed, then add that group to a user as an additional group. (I know this is done on our site but I haven't actually done it myself so I don't know the details - maybe start by copying the existing group?). We have a couple of "tiers" of PM limits, so it seems like unless you have some reason to set a different number for a lot of different users, then this should work.