The short answer: one per artist
Enterprise Edition is powered by RLM Floating License system, in which a limited number of licenses are shared among a larger number of users over time. The amount of users able to use animBot at the same time (concurrent users) is limited by the number of licenses available on your server. That basically means you will need one license per artist that will use animBot at the same time in your team.
For example if you have 10 animators in your team that will use animBot, you will need 10 floating licenses. However in a scenario where you have 5 animators working only in the morning and another 5 working only in the afternoon, you will need 5 floating licenses at the most.
How many Maya instances are allowed per user?
Artists can launch as many Maya instances as needed, only one license will be checked out from the license pool per artist, regardless.
The license activates when a user launches animBot, either from clicking on the shelf animBot button or using a hotkey assigned to one of its commands. Launching Maya alone will not require an animBot license from the pool. Also good to mention that licenses are shared, meaning that a single user could have several Maya instances and that would only take one license from the pool. A license is released when all Maya instances that had checked out a license are closed or the user can explicitly unload animBot from animBot's menu/system/unload.
More details in the article Monitoring floating license usage
What if the number of artists changes over the course of a project?
It's possible to fluctuate the number of licenses over time, by purchasing license terms that goes from one week up to one year, in monthly increments, so you can get the exact number of licenses your team needs. It's also possible to increase the number of licenses and co-term with existing licenses.