Context & tooling used: https://ds9a.nl/hypernsec3/
A quick not too precise (+-10%) scan of number of signed records in various top-level zones. Note that NSEC estimations are particularly rough. Raw data is in zones.js.
For comparison, official numbers for some TLDs can be found on http://www.statdns.com/. Even more numbers are on http://rick.eng.br/dnssecstat/.