Theoretically you can have as many users as the user ID space supports. To determine this on a particular system check out the definition of the uid_t type. It is usually defined as unsigned int or int meaning that on 32-bit platforms you can create up to almost 4.3 billion users.

How many groups can be created in Linux?

The maximum number of groups a user can belong to on UNIX or Linux is 16.

Can we add multiple users in Linux?

Adding a single new user to a Linux system can be achieved through the useradd command. But system admins often get request to add many users. So Linux provides a different to do a bulk addition of many users to a system. This is the newusers command.