Is there a way to wear more? I want to put on a mask but it won't let me

A GA medal will allow you one extra slot while activated, and you can also use Ooh La Layers to get up to three extra slots per layer (that expire in a month).

What's a GA medal?

EDIT: Oh gold account haha. Thank you :)

A Gold Account Medal.

You can buy them in user shops, or in the cash shop.