There should be an email notification sent to  admins when a TeamRaiser registration limit has been reached. 

Currently, only a message displays on the TeamRaiser participant type selection page if registration is full.  There is no way for an admin to know unless they check it continually.  Not ideal when using it for multiple events simultaneously.

Also the "registration is full" message can't be edited at the TeamRaiser level, only system-wide in the Message Catalogue.