You will have to buy a 500-user license. The number of app-licensed users has to be equal to or greater than the number of Confluence or Jira-licensed users.
There are two reasons for this:
- The Atlassian Marketplace doesn't allow apps to have lower license tiers than the host license.
- If it were possible to offer app licenses for smaller numbers of users, the app licenses themselves would need to be significantly more expensive. Otherwise, app development and high quality support would not be sustainable.