Yes, Solid Security works with Multisite installations, and it must be Network Activated.
WordPress Multisite is a feature that allows you to create and manage multiple websites from a single installation of WordPress, making it easier to keep everything organized and consistent. Imagine having a "network" of interconnected websites controlled from one central dashboard. This network shares a file system and database and is typically variations of the same domain.
How many site licenses will I need?
You will only need one license if you have one parent site with multiple sub-sites. If you only have one parent site with multiple multisites, you will only need one license. However, if you need to license the plugin on multiple domains, you will need a license for each domain (site) the plugin is installed.
Note: It's not possible to edit the settings of the child sites as all settings are network activated and propagate down to the child sites from the parent site settings.
How to ensure that Solid Security works on your Multisite installation?
To ensure compatibility, we recommend setting up a staging environment of your Multisite installation first and testing the plugin there. This allows you to identify and address any issues before implementing changes on your live sites.
We also recommend creating regular backups of your sites before making any significant changes as a precautionary measure.