Visit the official Xbox Gamertag Change page. This is the best method for seeing exactly how a suffix might look if the name is already taken.
Since the major overhaul of the naming system in 2019, the rules for checking and claiming names have changed significantly. Here is everything you need to know about checking availability, the current 12-character limit, and how the suffix system works. How to Check Xbox Gamertag Availability gamertag check xbox