Is there a limit for the amount of coins you can keep? What happens if you gain more than 99,999 ?
6 digits can fit in the black area. It looks like the font size gets smaller to compensate for the additional digit.
I don't think there is a reasonable limit for coins, given that you're able to purchase 450,000 in the largest bundle.
The player cannot have more than $999,990 on most versions; once that amount is reached, all money collected won't affect their total.
On the iOS and Android versions it is possible to earn up to $9,000,000!