Gramps has no hard limits on the size of a database that it can handle. Starting with 2.0.0 release, Gramps no longer loads all data into memory, which allows it to work with a much larger database than before. In reality, however, there are practical limits. The main limiting factors are the available memory on the system and the cache size used for BSDDB database access. With common memory sizes these days, Gramps should have no problem using databases with [[Gramps PerformanceGramps_Performance#JohnBoyTheGreat_2019-12.2C_version_5.1.1|tens of thousands Millions of people]].