You can only collect a limited number of coins from races per day. You can collect a maximum of 300 coins per day (or 600 coins if you are subscribed to the Gold Pass). You can see if you have reached this limit on the screen that shows the coins collected after a race.
There is no limit to the number of coins that you can collect from Coin Rush, tour gifts, Log-In Bonuses, the Coins Aplenty event, or challenge completion per day.
The coin limit resets each day at 6:00 AM UTC (11:00 PM PDT/10:00 PM PST).