So im helping Echoen she commissions a lot and has multiple ocs and there are probably more then 300 commissions from her on this website.
So the idea was to make one tag that would all group them together.

So my question is do i make a copyright tag for that?

And if so the tag echoen already exists but is a character tag and i cant change that my self because it has to many posts under it.

Commissioners are not tagged. Precisely because it'd get mixed up with artist/character tags. Feel free to make a Set, though.

Agreed with Genjar, commissioners don't get copyright tags, only the individual characters they own. Creating a set would be the best method of organising her commissions.

If all of her characters have tags including the suffix _(echoen) (which is preferred, especially if the characters' names are not unique), all of them can be found with the search *_(echoen).