I have noticed a bug - when the Count is called for more than one property in the REST API, the count amount comes for the first property and the second property just inherite the same value.
You can try it by yourself for example from this endpoint: /data/UsersRating/47589812-24DA-4B56-8672-3B53F79C63C6?property=Count(commentChild)%20as%20commentsNumber&property=Count(likes)%20as%20likesNumber
App ID: F7217991-455B-46B8-886F-505F8B71BEBA
You can not aggregate more than relation column, you need to do it in separated API calls
But I can propose you to create a custom API Service which will run this requests and then return to the client combined data.
The only issue with that is that to get 4 different counts from relations I need to trigger 4 separated calls (from the service level) which takes more time - but if that is the only way to achieve that then I will stick to it.