There is no limit on a custom code that how much app users hits on the daily basis. However there is a limitation on number of request per second per App.Your app has a burst limit of 40 request per second. Also your custom code request should get executed in 10 seconds of threshold.
If your app has a higher number of concurrency, you have to move in our enterprise plan for the same.
Let us know if you have more question for us.