Using last 2 months.

Sale 1:

Sold for $22 on Marketplace.tf

https://marketplace.tf/items/30920;5;u3009

https://backpack.tf/item/6475857513

22/1.98 = ~11.1111111111111

X = 11

Sale 2:

Sold for 8 keys in an auction

Seller: https://backpack.tf/profiles/76561198089253101#!/compare/1521676800/1521763200

Buyer: https://backpack.tf/profiles/76561198234128068#!/compare/1521676800/1521763200

X = 8

Sale 3:

Bulk for Lantern Toe

Seller: https://backpack.tf/profiles/76561198133348135#!/compare/1525478400/1525564800

Buyer: https://backpack.tf/profiles/76561198309838525#!/compare/1525478400/1525564800

Sellers:

11 keys:

https://i.imgur.com/DETMk3L.png

https://backpack.tf/item/6684713809

12 keys:

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

https://backpack.tf/item/6481345534

And a seller at ~8 keys worth on the SCM:

https://i.imgur.com/et3rg3S.png

Sales at:

8

11

Would exclude 8 normally as it sold in an auction for a fair bit less than the other sale, but the SCM seller supports 8 being in the range.

Going with 8-11.