Potatoes are award based on 1 potato per minute per game if you lose, or one potato every half minute if you win (x2 or x2+1 that of losing team). Note the minutes used in this calculation are minutes that you have played rounded down (if the game went on 4 minutes but you joined and played just 2.5 minutes, you'd get 2 potatoes if you lose, or 5 if you won).
Why did I only get zero potatoes?Edit
TL;DR: You need to shoot someone within the last 60 seconds of gameplay to earn potatoes.
After finishing a game players may occasionally find they were award zero potatoes, even though they were active. The reason for this is that you are not awarded any potatoes for a game if you do not shoot anyone within the last 60 seconds of a game-play (starting 30 seconds after round starts[Confirm]). This most commonly occurs when a player is defending a VIP or a Flag by focusing on conjuring a barrier, or when the round ends while you are setting up a sneak attack (or are just lost).
Why is the currency potatoes?Edit
All Atelier 801 games to date use food as currency, so part of the reason they picked a food item was to continue this motif.
It is not known why they chose potatoes[Confirm], but it could be a reference to army soldiers eating a lot of potatoes in WWI/WWII, as well as peeling potatoes as part of KP duty (or mess duty). This can be seen in many older American comic strips that had a military theme around the WWII era, such as Beetle Bailey.