Apps are billed based on the number of users in your Atlassian product.
For Jira 7.0 or later, the app tier should match the maximum tier of the licensed Jira products on your instance. For example, if you're running Jira Software (500 users) and Jira Service Management (25 agents) on the same instance, you should purchase the 500-user tier for apps.
For versions of Jira prior to 7.0, the app tier should match the licensed user tier for Jira. Even if fewer users want to use the app than your Jira license, the two licenses should match exactly.
Note: While this app has features specific to Jira Service Management and Jira Software, the app is technically available across the whole Jira instance. Therefore the above guidelines for the license tier still apply.