Using last month.

Sale 1:

Sold for 8 keys

Seller: https://backpack.tf/profiles/76561198382589541#!/compare/1530403200/1530489600

Buyer: https://backpack.tf/profiles/76561197970538347#!/compare/1530057600/1530748800

Bot.tf confirms: https://i.imgur.com/sQPzm1T.png

X = 8

Sale 2:

Sold for 8 keys

Seller: https://backpack.tf/profiles/76561198382589541#!/compare/1531612800/1531699200

Buyer: https://backpack.tf/profiles/76561198259589902#!/compare/1531612800/1531699200 (nsfw)

Untraceable. Bot.tf confirms, but it is unreliable. https://i.imgur.com/sQPzm1T.png

Key histories below.

https://backpack.tf/item/7001218681

https://backpack.tf/item/7001218698

https://backpack.tf/item/7001218716

https://backpack.tf/item/7001218723

https://backpack.tf/item/7001218730

https://backpack.tf/item/7001218740

https://backpack.tf/item/7001218752

https://backpack.tf/item/7001218760

X = 8

Sale 3:

Sold to a bot for 5 keys

Seller: https://backpack.tf/profiles/76561198348859145#!/compare/1531094400/1531267200

Buyer: https://backpack.tf/profiles/76561198382589541#!/compare/1531180800/1531267200

Untraceable. Bot.tf confirms, but it is unreliable. https://i.imgur.com/sQPzm1T.png

Key histories below.

https://backpack.tf/item/6989386305

https://backpack.tf/item/6989386324

https://backpack.tf/item/6989386345

https://backpack.tf/item/6989386366

https://backpack.tf/item/6989386373

X = 5

Sale 4:

Sold to a bot for 5 keys?

Seller: https://backpack.tf/profiles/76561198214686512#!/compare/1530144000/1530403200

Buyer: https://backpack.tf/profiles/76561198382589541#!/compare/1530316800/1530403200

Untraceable. Bot.tf confirms, but it is unreliable. https://i.imgur.com/sQPzm1T.png

X = 5?

Sale 5:

Sold for 4 keys + 2 ref to a bot?

Seller: https://backpack.tf/profiles/76561198166908431#!/compare/1530921600/1531267200

Buyer: https://backpack.tf/profiles/76561198312583107#!/compare/1531094400/1531180800

Untraceable, apart from the ref. Bot.tf confirms, but it is unreliable.

X = 4?

Sales 6 & 7:

Sold for $30 and $23.99 on marketplace.tf.

https://marketplace.tf/items/30844;5;u3005

Gonna end up being around 12 and 15, which is significantly higher than current sellers/sales. Excluding both.

Sale 8:

Bulk for Circling Heart Rotation Sensation

Seller: https://backpack.tf/profiles/76561198262802628#!/compare/1531872000/1531958400

Buyer: https://backpack.tf/profiles/76561198080179568#!/compare/1531872000/1531958400

Sellers:

10 keys:

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

https://backpack.tf/item/6427185124

14 keys:

https://i.imgur.com/FH1hTdY.png

https://backpack.tf/item/6266724124

20 keys:

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

https://backpack.tf/item/6258662506

https://backpack.tf/item/6253080009

https://backpack.tf/item/6488912040

Sales at:

5

8 (2x)

And untraceable sales at 4 & 5.

5 is an outlier, excluding.

Going with 8 flat.