Сколько хотите. Мы не ограничиваем количество файлов или папок, которые вы можете хранить в своём аккаунте MEGA.
However, large accounts containing millions of files and folders can slow down the login process and increase the memory footprint of our apps. That said, once you are logged in, having such a large account will not slow you down significantly.
We are continuously working on improving large account performance, specifically on reducing their memory usage.