Some of the following information is available easily in Data Centre's System Info, but it is not easily found in Cloud instances:

• Number of projects (All projects, Team-managed/Next-gen projects, and JSM projects)
• Number of issues
• Number of workflows
• Number of comments
• Number of Custom fields
• List of apps/plugins
• Attachment size

Workaround

• Go to https://<your site name>.atlassian.net/jira/settings/system/migration/home to see a partial list:
  

• Number of projects (All projects, Team-managed/Next-gen projects, and JSM projects)
  https://community.atlassian.com/t5/Jira-questions/How-many-projects-do-we-have/qaq-p/1591954
  The endpoint https://<site name>.atlassian.net/rest/api/3/search returns a Total field which tells you the total number of projects.
  Note: If the project contains the key-value pair simplified:true, that means it is Team Managed.

• Number of issues
  https://community.atlassian.com/t5/Jira-questions/How-to-get-all-issues-in-jira-using-rest-api/qaq-p/826281
  https://<site name>.atlassian.net/rest/api/3/search returns a Total field which tells you the total number of issues.

• Number of workflows
  https://developer.atlassian.com/cloud/jira/platform/rest/v3/api-group-workflows/#api-rest-api-3-workflow-search-get
  https://<site name>.atlassian.net/rest/api/3/workflow/search returns a Total field which tells you the total number of issues.

• Number of comments
  Number of comments: This one is not available out of the box, but you can use our REST API to retrieve the comments for each issue: https://developer.atlassian.com/cloud/jira/platform/rest/v3/api-group-issue-comments/#api-rest-api-3-issue-issueidorkey-comment-get
  This endpoint does not return a Total field

• Number of Custom fields
  https://community.developer.atlassian.com/t/jira-cloud-rest-api-for-next-gen-projects-fetching-all-custom-fields-for-specific-projects/41994
  https://<site name>.atlassian.net/rest/api/3/field

• List of apps/plugins
  Not exportable - must be copy-pasted from the UI: https://<your site name>.atlassian.net/plugins/servlet/upm