I have multiple Wordpress installations on MAMP, which all have their own file structure (wp-admin, wp-includes etc.).
Because they are all development sites, it doesn't make sense to include the whole wordpress installation for every site.
Is it possible to only have one instance of Wordpress, and somehow link the correct wp-config.php and wp-content folder to each site, to reduce disk space?
There are different ways to organize it. You could use a single location with core files and implement boot that points to different tables conditionally. You could implement multiple locations and symlink a single copy of core around.
Practically though you should consider which issue are you really solving with this? The disk space is insignificant. The versioning is better handled by proper tools for it like Composer.
In my practice I use a single anything-goes dev install for plugins/themes and make extensive use of on-the-fly plugin and theme switching. Any site-level projects just get their own separate install, versioned with Composer.
Yes it is possible but it is not extremely trivial as you can't just symlink the directories and some files need to be excluded. but in any case the gain on a development machine will be hard to notice. On a development machine you usually have sites in different stages of development, some of them might not be compatible with the latest wordpress version, some of them will never be compatible.
It is just doesn't make sense from economic point of view. If it takes you 5 minute to install new wordpress and 1 minute to upgrade while creating the environment and scripts take you 3 hours, you will have to create many new sites and do many upgrades before you will get even on your initial time investment. And this before taking into account some upgrades breaking sites without you being aware of that, and you discovering that only when you need to ship something right at that moment.
