One of the more common questions that comes up during OpenNMS troubleshooting is "how big is your installation" - which usually boils down to how many nodes/interfaces/RRDs a given installation is supporting.

It would be handy to present this information on the "About" page. I believe these would catch node and interface counts -

Node count

SELECT COUNT FROM node;

Interface count

SELECT COUNT FROM ipinterface;

Might need some logic in there to exclude rows that have been marked for deletion by vacuumd (if that even applies to nodes/interfaces).

For RRD count, a simple count of the number of RRD files in the RRD data directory should suffice.