When using the Built-in Policy - Default Logon Name or a copy of that policy, and when generating a name which is expected to have a unique number appended, a name and number are generated as expected.
If a User from this number series is deleted, the policy will re-use that number rather using the next highest number. This could result in reusing a name which was previously used by another Active Directory object.
This policy does not have a history or counter.
It consecutively searches for Users who meet the naming criteria, and then it will insert a new User into the series in the first available slot. This means that if live data changes, the end result of the name which is assigned will also change.