The actual RAM you need depends on many factors:
- plugins
- theme
- installed extensions (server and PHP)
- the current operation (editing images needs the most)
- translation, some translation files are really heavy
In a multisite, some resources are shared, the server setup for example is not duplicated each time you create a new site.
You can save some resources if you enable the built-in Opcache, check your theme and the plugins with Xdebug. There is not much you can do with translations.