It’s not a count, it is maximum.
E.g. you have a collection with ID property, ID is numeric.
You add a record and set ID to 999.
You add another record and set ID to 1.
When you use “Count”, you’ll get the value of 2.
When you use “Maximum”, you’ll get the value of 999.
If every time you add a new record, you put ID as Collection → ID → Maximum +1, then you will always have a higher value in a new record.
It works with all collections, not just Users.
There is a slim chance that 2 app users will add records to the collection at the same time. With this you may end with two similar IDs. But again, the chance is quite low.
I’d say that personally I’d go with unique ID generated based on 2 large random numbers 
Best regards, Victor.