This check determines whether the number of site collections in the farm is in accordance with Microsoft recommendations.
SharePoint site collections contain at least one top-level site and can contain additional sites below the top level. Site collections are associated with parent web applications and are in those applications’ content databases. Site collections cannot be stored across multiple content databases.
The number of site collections that can be present in web applications or in the farm depends heavily on the capacity of server infrastructure, but Microsoft provides tested limits for each SharePoint version.
For SharePoint 2013 and 2016, the maximum recommended number of site collections in a farm is 500,000 with a Personal Site template and an additional 250,000 with different templates. This means the theoretical limit of site collections in a SharePoint farm is 750,000.
Keep the number of site collections below the recommended limit. If the number goes over, SharePoint’s performance can degrade.
Additional information can be found in the following articles: