Get a certain amount of coins at the casino and it'll trigger something.
But an exact amount or can I go over that certain amount?
The amount or more, mate. I seriously doubt the dev is that sadistic for it to be an exact amount.
i have 12k coins and no event has been triggered am i close to the amount needed or off
Check the quest board in the Rustboro city ranger guild, there should be a new quest there that asks you to investigate the Mauville city game corner. If you don't see the quest after collecting 12k coins then look again after you reach 15k coins.
