The DS3 Wiki states there are 15 total slabs possible.
On my new-game character I have 5 fully upgraded weapons and 10 slabs, but I haven't defeated Midir, so Shira couldn't have given her slab to me yet.
My fully upgraded weapons are
- The Rotten Ghru Dagger
- The Lothric Sword
- The Hollowslayer Greatsword
- The Exile Greatsword
- And the Four-Pronged Plow
If I kill Midir I could have 16 fully upgraded weapons on this play through. How is that possible?