Sale 1: Trade back.

https://backpack.tf/profiles/76561198050114720#!/compare/1551312000/1551398400

https://backpack.tf/profiles/76561198316682864#!/compare/1551139200/1551398400

https://backpack.tf/profiles/76561198316682864#!/compare/1551139200/1551484800

https://backpack.tf/profiles/76561198050114720#!/compare/1551312000/1551484800

Sale 2: ~18 Keys Pure.

https://backpack.tf/profiles/76561198316682864#!/compare/1551744000/1551916800

https://backpack.tf/profiles/76561198044684270#!/compare/1551744000/1551830400

Sale 3: Bulk.

https://backpack.tf/profiles/76561198001812215#!/compare/1551484800/1551571200

https://backpack.tf/profiles/76561198125086452#!/compare/1550966400/1551571200

Sale 4: Untrace.

https://backpack.tf/profiles/76561198020966788#!/compare/1553904000/1553990400

https://backpack.tf/profiles/76561198342161127#!/compare/1553904000/1553990400

Sale 5: Untrace.

https://backpack.tf/profiles/76561198049029019#!/compare/1555545600/1556409600

https://backpack.tf/profiles/76561198082061316#!/compare/1555545600/1556409600

Sale 6: Untrace.

https://backpack.tf/profiles/76561198062711407#!/compare/1557964800/1558051200

https://backpack.tf/profiles/76561198080179568#!/compare/1557964800/1558051200

Sale 7: Bulk.

https://backpack.tf/profiles/76561198337832751#!/compare/1555891200/1555977600

https://backpack.tf/profiles/76561197994697258#!/compare/1555891200/1555977600

Mp.tf Sales:

https://marketplace.tf/items/1162;5;u3001

Sale 8: Low.

Apr 2

$21.00 / $1.79 = 11.73 keys

Sale 9: QS.

Apr 12

$28.85 / $1.77 = 16.30 keys

Sale 10: QS.

Apr 22

$29.40 / $1.79 = 16.42 keys

Sale 11: ~20 Keys.

Apr 25

$36.00 / $1.79 = 20.11 keys

Sale 12: ~22 Keys.

May 12

$38.99 / $1.8 = 21.66 keys

Including Sales: 18/20/22, calling 22 an outlier.

Going with 18-20 keys.

Thanks to scourge for the premium!