Sale 1:

Sold for 12 keys

Seller: https://backpack.tf/profiles/76561198386441509#!/compare/1516320000/1516406400

Buyer: https://backpack.tf/profiles/76561198155075832#!/compare/1516233600/1516406400

X = 12

Sale 2:

Sold with ~3.5 keys in items for Blizzardy Dixie (15)

Seller: https://backpack.tf/profiles/76561198038751270#!/compare/1515715200/1515801600

Buyer: https://backpack.tf/profiles/76561198002757046#!/compare/1515715200/1515801600

X = 11

Sale 3:

Sold for 10 keys + 1.44 ref

http://www.tf2outpost.com/trade/29468984 - No B/O

Seller: https://backpack.tf/profiles/76561198121830870#!/compare/1515542400/1515628800

Buyer: https://backpack.tf/profiles/76561198038751270#!/compare/1447142400/1515628800

X = 10

Sale 4:

Sold for 9 keys

Seller: https://backpack.tf/profiles/76561198055774847#!/compare/1514937600/1515196800

Buyer: https://backpack.tf/profiles/76561198003926663#!/compare/1515110400/1515196800

X = 9

Sale 5:

Sold for 8 keys to a bot

Seller: https://backpack.tf/profiles/76561198452011322#!/compare/1514851200/1514937600

Buyer: https://backpack.tf/profiles/76561198386441509#!/compare/1514851200/1514937600

X = 8

Sale 6:

Quicksold-ish for 8 keys + 11 ref

Seller: https://backpack.tf/profiles/76561198410583537#!/compare/1515456000/1515542400

Buyer: https://backpack.tf/profiles/76561198305578110#!/compare/1515456000/1515542400

X = 8

Sale 7:

Sold for Starstorm Slumber Wing Mann

Seller: https://backpack.tf/profiles/76561198305578110#!/compare/1515628800/1515715200

Buyer: https://backpack.tf/profiles/76561198143345427#!/compare/1515628800/1515715200

Should fit into the range pretty well.

Sellers:

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

10 keys:

https://backpack.tf/item/6488758146 - 3 days

https://backpack.tf/item/5554925178 - 1 day, lowered/re-listed over the course of a month

12 keys:

https://backpack.tf/item/3393917377 - 15 days

30 keys:

https://backpack.tf/item/3529210627 - 1 month

Sales at:

8 (2x)

9

10

11

12

Gonna give 8 the benefit of the doubt here, looks kinda low, both sales sold quickly. Excluding 8.

Going with 9-12.