Using last month.

Sale 1:

Sold for 11 keys

Seller: https://backpack.tf/profiles/76561198312583107#!/compare/1534464000/1534550400

Buyer: https://backpack.tf/profiles/76561198133868529#!/compare/1534464000/1534636800

https://backpack.tf/item/4254470743

https://i.imgur.com/ysC2MHN.png

Last set of keys on the seller’s end. Near ID shows a completely different set of keys, most likely because the buyer modified it, changing its ID around the same time the seller obtained the set of keys that match. The buyer obtaining it at the same time as other items are obtained support the modification part of my claim.

X = 11

Sale 2:

Sold for 10 keys

Seller: https://backpack.tf/profiles/76561198305676157#!/compare/1534204800/1534291200

Buyer: https://backpack.tf/profiles/76561198111123084#!/compare/1522195200/1534464000

https://backpack.tf/item/5802558026

https://i.imgur.com/ysC2MHN.png

X = 10

Sellers:

https://i.imgur.com/VCzmVTT.png

https://i.imgur.com/O8zXFaa.png

https://backpack.tf/item/4249921350 - 19 keys

https://backpack.tf/item/4441617174 - 20 keys

https://backpack.tf/item/4331796704 - 20 keys

https://backpack.tf/item/6304991887 - 24 keys

https://backpack.tf/item/4439487552 - 25 keys

https://backpack.tf/item/4317866301 - 27 keys

https://backpack.tf/item/5228677438 - 27 keys

Sales at:

10

11

Going with 10-11.