No. There is no limit to the number of Users created, or number logged in at any one time. Please see our article on creating new users. We do recommend each user have their own login (for traceability, permissions and rights). For ease, you can bulk manage users using profiles and a user can belong to more than one.
If your organisation uses SSO, you'll be able to allow access based on attributes - again, there are no limits to number of users in these configurations.