We don't know a way of doing this using Project GC (but some of the experts might be able to help there), but we have used a flag counter to achieve this with the one cache we have which gets a lot of cachers from different countries - SideTracked Cambridge Station:
https://coord.info/GC6V0YG
The flag counter has recorded cachers from 22 countries so far for this cache, but we have seen flag counters with far more than 22, so it should help to get you over 20.