While I discovered dragon priest masks in my first playthrough, I didn't discover the Labyrinthian connection (trying to keep spoilers to a minimum) until my second playthrough when my mage got sent to the area, and it had a real thrill of discovery. (Hooray for preventing myself from going to the UESPWiki for a while!)

I'm now in the process of building a house in the CK, and I'm considering having busts to mount the masks on the wall, as well as a quest that would explain why the previous owner knew about the masks and their number. However, I'd like a few honest opinions about whether revealing the final number of masks would be too much of a spoiler for a player using my house, or is it pretty much common knowledge now.

I think that there are 8 masks is common knowledge, but the 8 masks unlocking the final 9th mask isn't as well known.

Maybe have a bust similar to the one in Bromjuaar Sanctuary, where the 9th slot is hidden until you put the other 8 on there?

This pretty much. You could have a person point you to the last location of the guy with the wooden mask and that's how you find out about how many masks there are etc.

You could have a wall with ten busts, if the prior owner knew about the ninth mask, they would (probably) also know that the Wooden Mask was a requirement to collect all of them, or assume the Wooden Mask was another Priest Mask.

As for RP reasons, you could claim that the previous owner was an expert and in Ancient Nordic History, but you would have to fill your house with a bunch of other Ancient Nordic Trinkets

I found the wooden one early in the game quite by accident. Looked like a dragon priest mask. Has a strange aura about it, but I can't remember where I found it. It was laying on the floor of a dungeon or cave among a bunch of bones.

I found a nice way to explain the presence of the busts in the house (they're not in that altar thingie, though, I'm hanging them separately for a different look). I used a tie-in to the wooden mask without revealing where it is, or what the other masks are called. So, it will be a spoiler in the sense that players will learn that they're worth collecting, but little else. The house is almost done! Finishing up some quest dialogue, and that should be it. Thanks for the input!