I couldn't be universally charged enough to make a joke for this one...

Sale 1:

https://backpack.tf/item/8568919929

Sold for 11 Keys

https://backpack.tf/profiles/76561198038856461#!/compare/1591056000/1591228800

https://backpack.tf/profiles/76561198145677772#!/compare/1591142400/1591228800

Marketplace Sales

https://marketplace.tf/items/tf2/30658;5;u96

(The following histories can't be directly set to any of the following sales)

https://backpack.tf/item/8768717710

https://backpack.tf/item/8708513480

https://backpack.tf/item/7570963304

Sale 2:

Sold on May 16th for $25, Keys were $1.89 (13.23 Keys)

Taking 13 Keys (Rounding down)

Sale 3:

Sold on May 17th for $25, Keys were $1.89 (13.23 Keys)

Taking 13 Keys (Rounding down)

Sales 4-6:

Sold on May 19th x3 for $16.77, $25 & $26, Keys were $1.89 (8.87, 13.23, 13.76 Keys respectively)

Taking 9, 13, & 14 Keys

Sale 7-8:

https://backpack.tf/item/8610645233

Sold on Jul 10th x2 for $27.71, Keys were $2 (13.86 Keys)

Taking 14 Keys x2 (Rounding up)

Sale 9:

Sold on Jul 3rd for $25, Keys were $2 (12.50 Keys)

Taking 13 Keys (Rounding up)

Going with 11-14, excluding 9 as an outlier.