Bills have gotten introduced to keep it at 9, but are generally shot down by democrats. Most recent one (I think, this isn't the easiest to research) is here. See all the sponsors are Rs[1]

Part of the problem is it requires an amendment so you need a super majority.

Imo democrats are waiting until they have enough of a majority to tank the reputation hit court packing would bring, but then lock it to 15 after they do so.

[1] https://www.judiciary.senate.gov/press/rep/releases/grassley...