Can I restrict the number of ssh connections to my Linux box - by username or by ip or both?
To allow login only some users you can use AllowUsers keyword in /etc/ssh/sshd_config. To restrict a number of ssh connections You can use MaxSessions keyword.
AllowUsers jack joe MaxSessions 5
AllowUsers This keyword can be followed by a list of user name patterns, separated by spaces. If specified, login is allowed only for user names that match one of the patterns. Only user names are valid; a numerical user ID is not recognized. By default, login is allowed for all users. If the pattern takes the form USER@HOST then USER and HOST are separately checked, restricting logins to particular users from partic- ular hosts. The allow/deny directives are processed in the following order: DenyUsers, AllowUsers, DenyGroups, and finally AllowGroups.