Sorry but you cannot limit number of trial users this way. I have the same problem and looking for solution.
'Shared users' - do not limit allowed trial users, i've tested it and even with 1 shared user set in userprofile used as a trialuser profile there can be may logged trial users. I think it is because 'shared users' allow to share the same username, but trial users are different each time (T-mac-addresss username).