Amount of participants is grayed out and cannot be changed.

If you clicked a part to change the number of participants in it, the Amount box may be grayed out.

This is because it has been set to: people (same as the event participant amount). This works well when all participants will be participant in this particular part. This allows all parts to be updated automatically when you change the number of participants in your basic information.

You change this by changing the number from people (same as the event participant amount) to people.