You can get up to 75+100+125+150+175+200+(50+75+100+125)+(150+175+200+225)=825+350+750=1925 Pirate silver.
So if you don't have a event chest key, you have to get 75 silver in tower chests(a big rare chest, or a rare chest & a common chest, or 3 common chests) to complete 'Open an event chest once.'