I'm not seeing the -l option in man softwareupdate, but it ran without complaining in my one test (on a machine that needed no updates).

You should be able to grep the lines relating to software updates, then pipe to "wc -l"

Store that in a variable, then write the host name and update count to a file, presumable with an easily parsible delimiter (like "=" for instance).

Push the file to a central mutually accessible location, then cat the files.
From there you can create your report.

The important part is determining the unique character/s you want to grep for on the hosts, before writing the host specific reports. It's pretty easy to determine from the "man" output, and running the command on a few machines that actually need updates.