Sale 1:

Sold for Lantern Little Buddy (20)

Seller: https://backpack.tf/profiles/76561198258264645#!/compare/1551398400/1551484800

Buyer: https://backpack.tf/profiles/76561198119559226#!/compare/1551484800/1551571200

https://backpack.tf/item/264489571

X = 20

Sale 2:

Sold with 20 ref for 15 keys

Seller: https://backpack.tf/profiles/76561198292677605#!/compare/1546732800/1547251200

Buyer: https://backpack.tf/profiles/76561198149175706#!/compare/1546992000/1547251200

https://backpack.tf/item/338975192

X = 15

Sale 3:

Sold for 13 keys + 13 ref

Seller: https://backpack.tf/profiles/76561198254491860#!/compare/1548115200/1548201600

Buyer: https://backpack.tf/profiles/76561198258264645#!/compare/1548115200/1548201600

https://backpack.tf/item/264489571

X = 13

Other Sales:

https://backpack.tf/item/279871053 (#1)

https://backpack.tf/item/264489571 (#2)

#1

Bulk for Searing Pullover

https://backpack.tf/profiles/76561198131633162#!/compare/1550966400/1551139200

https://backpack.tf/profiles/76561198134680656#!/compare/1551052800/1552003200

#2

Sold with 6 keys for Strange Searing Airdog

https://backpack.tf/profiles/76561198119559226#!/compare/1551657600/1551744000

https://backpack.tf/profiles/76561198213234076#!/compare/1551657600/1551744000

Better to use on the Airdog

Sellers:

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

https://backpack.tf/item/279871053 - 17 keys

And another on scrap for ~15 keys

https://i.imgur.com/ReTB8W3.png

https://backpack.tf/item/264489571

Sales at:

13

15

20

20 is countered by sellers. Excluding 20.

https://i.imgur.com/pEEYJuO.png - buyers at 13+, excluding 13.

Going with 15 flat.