Xray is available for Jira Data Center and Jira Cloud.
When it comes to licensing, Xray follows Atlassian Guidelines for Marketplace Apps.
All information around Xray licensing and pricing is available at Atlassian Marketplace. You can use the price calculator to know exactly how much Xray is going to cost you:
As within any other app for Jira, the user tier or user count of the instance must match with Jira, this means, that if you have a 100 users license for JIRA you will need an 100 users license for Xray. You can learn more about this at Atlassian Guidelines.
If you have questions around pricing and licenses, you can reach out to sales@getxray.app or open a Product Information request at Xray Service Desk.