Licensed per user.
JetBrains AI Assistant requires that you already own a JetBrains IDE product with an active commercial subscription license.
Once a JetBrains AI Assistant Subscription has been purchased, it is active across all JetBrains products with an AI service.

I use the free version of an IDE. Can I use AI Assistant? AI functionality is currently available only to individual JetBrains customers or organizations with commercial licenses.

Subscription Plan – In order to use JetBrains AI, You must have a Subscription. Your Subscription gives You and any Affiliates authorized by You the ability to use JetBrains AI in accordance with Your Subscription Plan. Depending on Your Subscription Plan, You will be able to use JetBrains AI in certain JetBrains Products during the Subscription Period as specified in Documentation and subject to certain limits, that are described for every Subscription Plan on JetBrains Website.

You will be able to use JetBrains AI features during the Subscription Period as long as Your use is within the limits of Your Subscription Plan and limits of fair use determined internally by JetBrains. JetBrains may limit Your access to JetBrains AI if You exceed the limits of Your Subscription Plan or the internal limits for the fair use.