Can someone explain to me what is the meaning of this? Is it true that SME server have maximum number users/license?
Perhaps something form the past (Mitel days?). AFAIK SME Server does not have a licensing system and certainly not a license that is invoking a maximum on the number of users that can be used on it.
I suggest you raise a bug for it. We can then discuss (in that bug) whether to remove affected string and code as it seems present in a function in useraccounts.pm, the panel in which users are created and edited, but my guess is it will never be invoked using the current server-manager.