How do the total coins add up?
Every time I collect one or only the ones I'm holding at the end of the race? In the latter case, it would always take at least 5 races (5 * 10 = 50) to unlock something since holding coins is limited to 10.
I assume it's the first case though since I possibly unlocked a new part after a Grand Prix (= 4 races).
The number of coins added to your total is the number of coins that each local player has at the end of each race. This means that you can get a maximum of 10 coins per race with one player, but 40 coins per race with four players.
Although you can collect enough coins to unlock multiple parts in a single grand prix or VS match, you can only unlock one part at a time. You must then return the the menu after viewing the newly unlocked part to receive any further parts that you have enough total coins to unlock.
It's the coins collected at the end of the race. It can be possible that you have multiple unlocks in a row after a grand prix, for example if you start with 95 coins.
After GP4 it stops, but in one grand prix you have again a coin amount just under 50, so you'll unlock again in a row. If you quit or restart the grand prix after a few races, you still obtained some coins. Therefor it isn't necessary to get 40 coins every grand prix.
