Is 16GB of RAM enough for running Nextcloud, Plex, and MariaDB in Docker?

Yes, 16GB is enough for a typical home setup with a handful of users. Nextcloud and MariaDB aren’t memory hogs unless you’re syncing terabytes or running many concurrent connections. Plex uses more RAM during transcoding, but unless you’re doing multiple 4K transcodes at once, 16GB handles it. For light to moderate use, you’ll rarely hit swap.