This should be in the WhatsUp Professional forum - sorry.   I'm new to WhatsUp - installed on a client site.  A related question is when I click on the State Summary report for Current Group the Group Summary -> Devices Up shows only a few of the devices, while the Device Summary -> View All -> Devices Up shows even fewer.   Why?  Any non-invasive (you can't break anything doing it this way) answers?

Answering my own post!   For others that might be new to this (should there be a newbies forum?), here is the/one answer

Reports > System > Diagnostic

View the System Diagnostic Report < View All Results >

The Device Table Count will have an entry that states ‘The database value (312) is less than or equal to the desired maximum (1000).    That number (312) is the number of devices currently being monitored