To understand how many projects are using Xray the easiest way is to understand where Xray Entities are being used.
Just check which Issue Type Schemes have Xray entities (Test, Test Plan, Test Set, Test Execution and Sub-Test Execution) included and then check how many projects are using those Issue Type Schemes.
Learn more about Issue Type Schemes at: https://support.atlassian.com/jira-cloud-administration/docs/what-are-issue-type-schemes/