It is the number of users that may be registered within a plan.
For example, if you subscribed for a business plan for 50 users, up to 50 users including the administrator may use said plan. (It is also possible to increase users later.)
The difference between the number of users and contacts
"Contacts" is like an address book. By registering contacts, you may send and receive chat messages with them, and invite them or be invited by them to group chats.
The number of contacts is the number of users you can communicate with.
Can I chat with only one registered user?
One is the number of users for free plans and personal plans, and you can still chat with other users by getting adding them to your contacts.
The difference between the number of users and number of licenses
Under the business plan, you will first need to acquire licenses, and you will be able to register or delete users as long as you have enough licenses.
Example: Subscribe to the business plan with 50 licenses, and register 40 users (=Can register 10 more users)