How many primary groups can a user be a member of? A user can have only one primary group at a time. However, a user can temporarily change the user’s primary group, with the newgrp command, to any other group in which the user is a member.
Can user be member of multiple groups? Yes, a user can be member of multiple groups: Users are organized into groups, every users is in at least one group, and may be in other groups. Group membership gives you special access to files and directories which are permitted to that group.
How many groups can a user be a part of Linux? The maximum number of groups a user can belong to on UNIX or Linux is 16.
How many groups can you have in AD? No, it’s limited to 1015 (including nested groups) due to the size of a principal’s security token. Here’s an article that discusses AD limits, including group memberships.