I owe a couple drones, with the Mavic Pro, I have 4 batteries, each marked from one to four.
I keep them stored in fireproof Li-ion storage bags and in a metal box, in my photography closet, along with my photography gear.As someone already mentioned, personally I keep them at 25% or 50% charged. And, I also check on them monthly to make sure they aren't self-discharging.
And, I have two batteries, in each fireproof storage bag, but I better idea is to put each one in its own bag. And, rethinking now putting each one in its own fireproof container, rather then putting everything I have got into one big container. Always keep learning here.! I also put the battery that I am charging in the metal box, close the lid, and do not leave the house while charging.