Continually count the number of open files

From: Graham Perrin <grahamperrin_at_gmail.com>
Date: Tue, 12 Sep 2023 06:38:35 UTC
Can anything like systat(1) present a count, continually?

I'd like to monitor, after log in to Plasma (X11), in connection with 
<https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=273669>.