How many Titantite Slabs are in Dark Souls 3?

The only way this might have happened is if another player dropped a +10 weapon for you. You can only transfer those if you already have a fully upgraded weapon (+10 with regular Titanite, or +5 with Titanite Scale).

So for example, if you already had the +5 Hollowslayer Greatsword, someone would be capable of dropping another +10 weapon.

I think I figured it out.

Shira drops it if you kill her in the ruins at the end of the world but your character hasn't yet defeated Midir.

This wiki page states that she has a guaranteed drop rate of the slab if she hasn't yet awarded it to you.

I'm guessing that Midir's defeat will not yield a Titantite Slab from Shira.

Update

That is exactly what happened.