Is the only way to get these upgrades by purchasing them from Piero?

I've got the Arcane Assassin pack that gives me a whale statue that (allegedly) gives me another bone charm slot, but instead of giving me one I can actually *use* it just gives me a grey slot, but Piero doesn't sell 5 upgrades, he only sells 4.

I don't know if this is a bug or not and I've not found anything in game that actually unlocks the 8th slot for me to equip an extra bone charm.

I got the acrobatic killer pack with the hagfish statue, got all of pieros upgrades and im at 7 out of 8 slots

Sounds like an oversight. Either the statues are supposed to unlock the slot for use or Piero is supposed to sell one additional upgrade.

I have three of the DLCs and can have 9 charms equipped if I buy the upgrades from Piero. On X-Box I believe I'll be able to have 10 as I have all the statues and can buy the upgrades whenever with the saves I'm keeping.

Well, I figured out that I'm just a boob.

Looks like, by default, the game only gives you six slots, 3 of which you buy from Piero. The Whale Statue gave me the 7th one. What was throwing me off was the greyed out slot at the end.

Basically? I'm an idiot and there's nothing wrong with the bone charm upgrades or the statue.