Working through mega strike as my whirlwind suggestions get left open until they’re invalid :)

Using last month

Sales 1 & 2:

Sold for $20 and $9.99 on Marketplace.tf

https://marketplace.tf/items/30917;5;u3010

20/1.98 = ~10.1

9.99/1.96 = ~5.1

X = 10

X = 5

Sale 3:

Sold for 6 keys

Seller: https://backpack.tf/profiles/76561198121830870#!/compare/1524873600/1524960000

Buyer: https://backpack.tf/profiles/76561198069580305#!/compare/1524873600/1524960000

https://i.imgur.com/imu2wFL.png

https://backpack.tf/item/6224575038 (screenshot and key history in case it changes later today)

X = 6

Sellers:

6 keys:

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

https://backpack.tf/item/6458784622

8 keys:

https://i.imgur.com/L4T1cSo.png

https://backpack.tf/item/6577075458

Sales at:

5

6

10

10 is an outlier, excluding 10.

Going with 5-6.