Return # of items in a Repeating Group

Is there an Operator that returns the # of items in a Repeating Group?

I’m not sure if i understand what you mean exactly by number of items in the repeating group.
But if you mean the number of items in the Data source list of the repeating group then you can simply use the :count method on the list.
e.g. Search for MyThing: count will give a number which is the number of items in the MyThing list.

Will this take in consideration all the constraints on the Data Source? or this returns the whole database of that Type?

Yes tt will take into consideration the contraints.