Thanks go to The Real Slim Strange.

https://backpack.tf/profiles/76561198311498888#!/compare/1625529600/1626134400 - Bulk.

https://backpack.tf/item/9539158150 - Untraceable.

https://backpack.tf/profiles/76561198067651658#!/compare/1630368000/1630454400 - 8 keys 17.77 ref, rounding it to 8.

https://files.catbox.moe/04w3qd.JPG - 13 keys 25 ref, rounding it to 13.

Mp.tf: https://marketplace.tf/items/tf2/31153;5;u3005

Sold thrice for 37.99 USD in the last 3 months.

37.99/1.81 = 20.98

37.99/1.82 = 20.87

37.99/1.84 = 20.64

Rounding all of these to 21.

Sales:

8

13

21

21

21

8 is the biggest outlier and isn't supported by current buyers.

https://gladiator.tf/listings/6153dc3287798a721155be1e

Since there were 2 sellers for 13 keys until recently, it's safe to assume that despite creating a big range, 13 is usable.

Taking 13-21 keys.

https://gladiator.tf/listings/616f0650a4ac03428e1a1058

RIP