Using last month.

Sale 1:

Sold for 8 keys

Seller: https://backpack.tf/profiles/76561198086076383#!/compare/1523318400/1523404800

Buyer: https://backpack.tf/profiles/76561198105928538#!/compare/1523318400/1523404800

X = 8

Sale 2:

Sold for 7 keys

Seller: https://backpack.tf/profiles/76561198182213262#!/compare/1523059200/1523145600

Buyer: https://backpack.tf/profiles/76561198303040717#!/compare/1522800000/1523145600

X = 7

Sale 3:

Sold for Collector's Pro. KS Brass Beast

http://www.tf2outpost.com/trade/29617539 - B/O of 10

Seller: https://backpack.tf/profiles/76561198092489052#!/compare/1523750400/1523836800

Buyer: https://backpack.tf/profiles/76561198272058312#!/compare/1523664000/1523750400

Brass beast has a sale at ~8 keys worth on the SCM, and a seller at 7 on the classifieds. Should fit in the range.

Sale 4:

Bulk for Lantern Chieftain's Challenge

Seller: https://backpack.tf/profiles/76561197997283715#!/compare/1524182400/1524268800

Buyer: https://backpack.tf/profiles/76561198062749575#!/compare/1524182400/1524268800

Sale 5:

Bulk-ish for Circling Heart Airdog

Seller: https://backpack.tf/profiles/76561198062749575#!/compare/1524182400/1524268800

Buyer: https://backpack.tf/profiles/76561198348422982#!/compare/1524182400/1524268800

Sellers:

9 keys:

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

https://backpack.tf/item/4984095921

11 keys:

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

https://backpack.tf/item/6523729138

12 keys:

https://i.imgur.com/20jA4a2.png

https://backpack.tf/item/6508799871

20 keys:

https://i.imgur.com/mZurV7j.png

https://backpack.tf/item/4960641708

26 keys:

https://i.imgur.com/Dm0Xawd.png

https://backpack.tf/item/6341971717

Sales at:

7

8

Going with 7-8.