Gnarly has 8 dragon coins, there might be another level that has more than 5, but that is the first that comes to mind. Then there is a single dragon coin in Chocolate Secret [CS].

When 5 dragon coins are collected in a level, you receive a 1up, but also the dragon coins will no longer appear in the level. So the level with 8, nothing tracked changes after the 5th coin (you will receive a 1up for each additional coin over 5 you pick up), and in CS the coin will respawn if you enter the level again.

Once you get 5 dragon coins, you don't need to collect anymore because you have no reason to do that.

no dragon coins respawn once you collected 5 (even if there are more than 5 in a stage)