Carnival game

I rarely do redstone and I dont need it to be small:

Goal: a game that when 9 minecarts are in the [green 3] chest then a prize from [purple 2] chest is output into [blue 1] chest

[blue 1] is accessible to the minigame player, the rest would be hopefully underground

The issues that i dont know how to solve:

1: Having a specific output size, as it is right now it will just keep going till [green 3] is emptied

2: How to empty [green 3] automatically after prize size is output

3: make it uncheesable as it’s going in a public server

How its working now:

The redstone is working, the concept is there the working part is not

Any help is appreciated even if you can only solve 1 piece

If you have any questions I’ll answer in comments, unedited photos in img2