I know 9999 coins maxes out the coins display, but do coins accrue beyond the 9999 even if they don't display? For example, if I have 9999 coins, find 10, and spend 10, will my display register as 9999 or 9989? Trying to determine whether I would immediately need to spend down the number or else risk wasting coins.
9999 coins is a hard limit.
To test this, I got 9999 coins from Luigi’s Balloon Hunt. I then continued to collect more coins, which did not increase the counter.
Spending 10 coins to try hunting another balloon left me with 9989 coins. After collecting past the cap again, buying the skeleton costume for 9999 coins left me with 0 coins.