As @Asunez stated, you can purchase keys from the in-game menu and changing the quantity in the box next to the buy button.
On top of that, it's cheaper to purchase your keys through the game store as opposed to the market as the keys are more expensive ($2.69 USD^) than their worth ($2.49 USD).
Anyway, here goes:
Since a Chroma Case Key is a commodity, you can buy multiple items by selecting the big Buy... button, and changing the quantity you would like to buy, as indicated by the big red circle I have drawn with a touch-screen laptop.
^ Disclaimer: Market prices are expected to fluctuate and this statement may be false at some periods of time. This information is correct at the time of writing.
From within the game you can buy multiple keys at once.
As far as I remember there are two ways to do so:
- From the main screen, there is a small frame on top where you can select a key and choose the amount of keys you want to buy.
- Try to open a case from in-game inventory. If you do not have any keys a popup will show asking you to buy keys. There should also be a quantity chooser there.