Using last month.

Sale 1:

Sold for Pro. KS Collector's Big Earner (~9)

Seller: https://backpack.tf/profiles/76561198178103105#!/compare/1523923200/1524182400

Buyer: https://backpack.tf/profiles/76561198314160142#!/compare/1524096000/1524182400

X = 9

Sale 2:

Sold for 7 keys + 3 ref

Seller: https://backpack.tf/profiles/76561198305676157#!/compare/1523577600/1523664000

Buyer: https://backpack.tf/profiles/76561197966445303#!/compare/1481500800/1523664000

Untraceable (only 1 ref on the seller's end shows), but bot.tf confirms: https://i.imgur.com/hD0vdCp.png

X = 7

Sale 3:

Sold for 7 keys + 3 ref

Seller: https://backpack.tf/profiles/76561198305676157#!/compare/1523664000/1523750400

Buyer: https://backpack.tf/profiles/76561198092449612#!/compare/1523664000/1523750400

Untraceable, bot.tf confirms: https://i.imgur.com/hD0vdCp.png

X = 7

Sale 4:

Sold for 6 keys + 26 ref

Seller: https://backpack.tf/profiles/76561198305676157#!/compare/1523923200/1524009600

Buyer: https://backpack.tf/profiles/76561198232157836#!/compare/1523923200/1524009600

Bot.tf confirms: https://i.imgur.com/hD0vdCp.png

X = 7

Sale 5:

Sold for 7 keys

Seller: https://backpack.tf/profiles/76561198115599217#!/compare/1525305600/1525392000

Buyer: https://backpack.tf/profiles/76561198301880376#!/compare/1525305600/1525392000

X = 7

Sale 6:

Bulk for Circling Heart Prehistoric Pullover

Seller: https://backpack.tf/profiles/76561198090560650#!/compare/1523318400/1523664000

Buyer: https://backpack.tf/profiles/76561198037631433#!/compare/1523577600/1523664000

Sellers:

9 keys:

https://i.imgur.com/txS7qAe.png

https://backpack.tf/item/6203766088

https://i.imgur.com/lMqxub9.png

https://backpack.tf/item/5424440972

http://www.tf2outpost.com/trade/29633819

10 keys:

https://i.imgur.com/fV90B2s.png

https://backpack.tf/item/4426399187

12 keys:

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

https://backpack.tf/item/4412820186

47 keys:

https://i.imgur.com/KFeKu8x.png

https://backpack.tf/item/4979195859

Sales at:

7 (4x)

9

Excluding 9, as it doesn't appear to be too common, and isn't pure, while the 4 sales at 7 are. Also, the value of the Collector's item can be a bit tricky to determine, so it's best left excluded anyways.

Going with 7 flat.