Using last month.

Sale 1:

Sold for 8 keys

Seller: https://backpack.tf/profiles/76561198312583107#!/compare/1531353600/1531440000

Buyer: https://backpack.tf/profiles/76561198268798857#!/compare/1531267200/1531440000

Bot.tf confirms: https://i.imgur.com/BUa41B7.png

X = 8

Sale 2:

Sold for 8 keys

Seller: https://backpack.tf/profiles/76561198335791307#!/compare/1532044800/1532131200

Buyer: https://backpack.tf/profiles/76561198254039408#!/compare/1532131200/1532304000

Bot.tf confirms: https://i.imgur.com/78uTHeS.png

X = 8

Sale 3:

Sold for 7 keys

Seller: https://backpack.tf/profiles/76561198833317554#!/compare/1532390400/1532476800

Buyer: https://backpack.tf/profiles/76561198146283733#!/compare/1531353600/1532476800

Bot.tf confirms: https://i.imgur.com/BUa41B7.png

X = 7

Sale 4:

Sold to a bot for 6 keys + 11.11 ref?

Seller: https://backpack.tf/profiles/76561198109904696#!/compare/1529798400/1529971200

Buyer: https://backpack.tf/profiles/76561198305676157#!/compare/1529884800/1529971200

Untraceable. Bot.tf confirms, but it is unreliable. https://i.imgur.com/BUa41B7.png

X = 6.5?

Sale 5:

Sold to a bot for 6 keys + 12 ref?

Seller: https://backpack.tf/profiles/76561198074971373#!/compare/1530835200/1530921600

Buyer: https://backpack.tf/profiles/76561198312583107#!/compare/1530835200/1530921600

Untraceable. Bot.tf confirms, but it is unreliable. https://i.imgur.com/BUa41B7.png

X = 6.5?

Sale 6:

Sold to a bot for 5 keys?

Seller: https://backpack.tf/profiles/76561198067973130#!/compare/1532304000/1532563200

Buyer: https://backpack.tf/profiles/76561198833317554#!/compare/1532217600/1532304000

2 keys are untraceable, but I highly doubt anything else would’ve been involved. Bot.tf confirms. https://i.imgur.com/BUa41B7.png

X = 5

Sale 7:

Sold to a bot for 5 keys + 8.11 ref?

Seller: https://backpack.tf/profiles/76561198321695483#!/compare/1530489600/1530921600

Buyer: https://backpack.tf/profiles/76561198335791307#!/compare/1530403200/1530489600

Untraceable. Bot.tf confirms, but it is unreliable. https://i.imgur.com/BUa41B7.png

X = 5?

Sales 8-11:

Sold for $24.90, $23.40, $23, and $20 on marketplace.tf

https://marketplace.tf/items/30845;5;u3005

All these sales are significantly higher than other sales/sellers. Excluding them.

Sellers:

8 keys:

https://i.imgur.com/0xFOVGC.png

https://backpack.tf/item/6286947861

~8.3 keys:

https://i.imgur.com/yPqgKX8.png

https://backpack.tf/item/6398757413

15 keys:

https://i.imgur.com/78uTHeS.png

https://backpack.tf/item/6315461609

https://backpack.tf/item/6252766244

Sales at:

5

7

8 (2x)

And untraceable sales at 5 and 6.5 (2x)

5 does not appear to be a common trading point, excluding 5.

Going with 7-8.