Allowing only a certain number of users in SAP at one time

Suppose I want to allow 10 SAP users onto the system at once, how can I restrict a 11th user from logging into SAP? Can I send him a message that he cannot log in now?

SAP has a user exit that is invoked during a SAPGUI login that allows for some processing logic. SAP discourages use of this for preventing a login. You may consider developing some application functionality (including messages) that exploits this user exit.