Using April onwards.

Sale 1:

Sold for $35 on Marketplace.tf

https://marketplace.tf/items/30919;5;u3004

https://backpack.tf/item/6550466738

http://www.tf2outpost.com/trade/29609986 - B/O of 25

35/1.97 = ~17.8

X = 18

Sale 2:

Sold for 12 keys

Seller: https://backpack.tf/profiles/76561198305676157#!/compare/1523836800/1523923200

Buyer: https://backpack.tf/profiles/76561198067200487#!/compare/1523750400/1523923200

Bot.tf confirms: https://i.imgur.com/SY2IIDH.png

X = 12

Sale 3:

Sold for 11 keys

Seller: https://backpack.tf/profiles/76561198305676157#!/compare/1524873600/1524960000

https://backpack.tf/item/6709439964 - Buyer can’t be identified

Bot.tf confirms: https://i.imgur.com/SY2IIDH.png

X = 11

Sale 4:

Sold to a bot for 11 keys + 2 ref

Seller: https://backpack.tf/profiles/76561198050930436#!/compare/1522800000/1522886400

Buyer: https://backpack.tf/profiles/76561198305676157#!/compare/1522800000/1522886400

X = 11

Sale 5:

Bulk for Green Energy Antarctic Eyewear

Seller: https://backpack.tf/profiles/76561198089675316#!/compare/1525392000/1525478400

Buyer: https://backpack.tf/profiles/76561198069813252#!/compare/1525392000/1525478400

Sale 6:

Bulk for keys

Seller: https://backpack.tf/profiles/76561197990998139#!/compare/1526515200/1526601600

Buyer: https://backpack.tf/profiles/76561198095236132#!/compare/1526428800/1526601600

Sellers:

18 keys:

https://i.imgur.com/ikYnfg0.png

https://backpack.tf/item/6466288468

Sales at:

11 (2x)

12

18

18 is an outlier, excluding.

Going with 11-12.