In Subaru’s case, Rem made multiple copies of his tracksuit but how they still exist after her name was eaten I guess doesn’t really make sense.
As for everyone else, it’s just that typical anime thing of keeping characters in the same outfit so you don’t need a design sheet for multiple outfits.
Well, it’s also an issue in the LN but given that only has ~10 illustrations per book, it’s not as noticeable.
On the plus side, Subaru gets an outfit change for the next part of the story after Priestella which he then keeps for the next few arcs without change…