Using last 2 months.

Sale 1:

Sold for 9 keys

Seller: https://backpack.tf/profiles/76561198048851745#!/compare/1521936000/1522022400

Buyer: https://backpack.tf/profiles/76561198135865374#!/compare/1521936000/1522108800

X = 9

Sale 2:

Sold for 8 keys

Seller: https://backpack.tf/profiles/76561198121830870#!/compare/1521676800/1521763200

Buyer: https://backpack.tf/profiles/76561197972160696#!/compare/1521331200/1521763200

X = 8

Sale 3:

Sold for 5 keys to a quickbuyer

Seller: https://backpack.tf/profiles/76561198166380094#!/compare/1523059200/1523318400

Buyer: https://backpack.tf/profiles/76561198121830870#!/compare/1523232000/1523318400

X = 5

Sale 4:

Sold for 4 keys to a quickbuyer

Seller: https://backpack.tf/profiles/76561198017911471#!/compare/1524873600/1525046400

Buyer: https://backpack.tf/profiles/76561198121830870#!/compare/1524960000/1525046400

X = 4

Sale 5:

Bulk

Seller: https://backpack.tf/profiles/76561198125082062#!/compare/1524441600/1524528000

Buyer: https://backpack.tf/profiles/76561198176388251#!/compare/1524441600/1524528000

Sale 6:

Bulk for Aus Scattergun

Seller: https://backpack.tf/profiles/76561198223561464#!/compare/1523145600/1523318400

Buyer: https://backpack.tf/profiles/76561198166380094#!/compare/1523059200/1523318400

Sale 7:

Bulk for Green Confetti Bear Necessities

Seller: https://backpack.tf/profiles/76561198115699456#!/compare/1521936000/1522022400

Buyer: https://backpack.tf/profiles/76561198125082062#!/compare/1521849600/1522022400

Sale 8:

Bulk for spelled stuff

Seller: https://backpack.tf/profiles/76561198399418133#!/compare/1520726400/1520812800

Buyer: https://backpack.tf/profiles/76561198115699456#!/compare/1520640000/1520726400

Sale 9:

Bulk for hats

Seller: https://backpack.tf/profiles/76561198254375254#!/compare/1522281600/1522368000

Buyer: https://backpack.tf/profiles/76561198129430133#!/compare/1521590400/1522281600

Sellers:

10 keys:

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

https://backpack.tf/item/6710585561

https://backpack.tf/item/6477893873

Sales at:

4

5

8

9

Sales at 4 and 5 went to a quickbuyer, excluding them.

https://i.imgur.com/glAwEyH.png - Quickbuyer at 8 is invalid, as they state they aren’t after wind effects.

Going with 8-9.