Assassin's Creed Mirage

Elixirs are your health potions. They work a lot like Estus Flasks from Dark Souls, in that you can only carry a certain amount at a time, and you have to top them up when you run out.

Now, this achievement just means to fill your stash with 10 elixirs. Not at once. Cumulatively. So no, you don’t have to be able to carry 10 at once or anything like that. You’ll no doubt get this one naturally, but if by some miracle you don’t, at the Hidden Ones Bureau in Harbiyah, there’ll be a chest opposite the Ahmed (who is part of the Banu Musa, a family of identical brothers who make tools for you) which you can use to replenish your stocks. If you don’t have a full allotment of elixirs, it’ll get filled up for you. Keep returning here - or looking out for them in the open-world - and fill your stash with 10 elixirs, and this achievement is yours.