The way I currently understand if for the Americas region is that; The highest 24 players will compete, however if 11 Mexico players are in the top 24 then the top 10 will compete and the place will be given to the 25th player if they are eligible based on the same rules.
This is the same for manufacturer except it's top 2 players for each manufacturer, so if 3 Nissan drivers are in the top 24, if all 3 enter the top 2 will join the race the 3rd will be dropped for the next highest eligible player of a different manufacturer.