My gamertag still has a number suffix even though the original gametag is not being used

There's no other way around that. As you stated, you would have to change it twice and pay both times to be able to do that.

Their are gamertags that are still registered from the system but are not searcheable anymore due to it being inactive. If the system still recognizes that tag, it will always provide you a suffix after it. If you want to have a gamertag without suffix, you can change it again and just make sure to double check before confirming if the system will provide suffix or not to know that this has not been used by anyone yet. But suffix is really not that bad since it will show up smaller in game, like being separated from the original tag.

It can take time for the system to recognize the change. So if it's still giving the suffix, then the system is still showing the gamertag as in use even though it has been changed. You'd have to wait for it to recognize that it's been freed up before you can use it without the suffix.

With the updated gamertag policy, you can still use it even though someone else has it, but you will be given a suffix to differentiate you from the original owner. Otherwise, as long as the tag is in use on the system, you cannot use the tag. The tag system is in use on other Microsoft services, so even if it is not in use on an Xbox, it may be in use elsewhere.

I checked and there is nobody that has the same user name as me

If it's giving you a suffix then its definitely in use.

Keep in mind the gamertag system doesnt see captilizations and spaces as making a gamertag unique so when changing your gamertag it would see "MyGamertag", "My Gamer Tag" and mygametag" all as the exact same gamertag.

When you are seaching manually to see if someone has a certain name or not you have to search for it EXACTLY as they have it otherwise itll say no one has that gamertag even though someone might actually have it but just with different captilizations or spacing.

For example lets say you are wanting the gamertag "MyGamertag" and you search to see if someone already has the gametag of "My Gamer Tag" itll say it doesnt already exist.

When you go to actually change your gamertag to "MyGamertag" it knows "My Gamer Tag" already exists and it will therefore put a suffix at the end of the gamertag you want.

Hopefully thats not too confusing but that's essentially how that works.

Tldr: if its giving you a suffix then someone DOES have the gamertag you want but could just have any number of combinations of changes between the captilizations and spacing.