To find out how many systematic reviews are in PubMed’s Systematic Reviews database subset, in either the PubMed search box or PubMed’s Systematic Reviews search box, type: systematic[sb]
To find out how many articles are in PubMed by Clinical Queries filter category, copy the filter category here and paste it into the PubMed search box
To find out how many articles are in an Ovid database, in the search box, type (don’t forget the dots!): docz.dz.
And lastly, to find just the free fulltext, full text [sb], retrieves citations that have a link to full text when no payment or subscription is required.