Following are few limitations about custom discord tags.
• Any Tag that is available for your username can be claimed by you. We cannot promise that a Tag will be accessible for you because each username+Tag combination must be unique.
• 4-digit digits between #0001 and #9999 are acceptable tags.
• If you select a customized Tag, it will be in place as long as your Nitro subscription is ongoing. Your Tag will be changed if your subscription expires. It won't simply revert to an old Tag because it wasn't don't save.
• You are only permitted to modify your Username or Tag so frequently. Try again in an hour if you encounter a rate-limited problem.