Xray is available for Jira Data Center and Jira Cloud.
When it comes to licensing, Xray follows the Atlassian Guidelines for Marketplace Apps.
You can review all the relevant information around Xray licensing and pricing in the Atlassian Marketplace listing. When accessing the site, note that switching between deployment types is necessary in order to reach the information pertaining to your specific model. Additionally, the site has an FAQ section at the bottom of the page with useful information.
You can use the Price tab to estimate the pricing for your site:
The app's user tier must match your overall Jira user tier. This is a mandatory rule defined by Atlassian directly that applies to all third-party apps, you can find that information detailed HERE. This means, that if you have a 100 users license for Jira then you will need an 100 users license for Xray.
If you have additional questions around pricing and licensing, you can reach out to sales@getxray.app or open a Product Information request at Xray Service Desk.