Piping jobs output

I have a number of shell scripts that scan various directories
updating files. They can be quite time consuming so I put
parts of the operations in the background and use "jobs" to
manage how many things I put in the background. With bash
this work beautifully but with FreeBSD's /bin/sh it's broken.

So here's how I've been doing things:

How I count the number of jobs I have in the background:
jobs | wc -l

In my scanning loop, to limit the jobs I create I have:
while [ "$(jobs | wc -l)" -ge 4 ]; do
  sleep 2
done

In /bin/sh the piping of jobs output doesn't seem to work.

This lists all the jobs:
jobs

This always returns 0:
jobs | wc -l

The just returns an empty line:
jobs | cat

I don't know if I've stumbled on a bug or feature. I haven't
seen anything that says I can't pipe the output of builtin
commands in sh. 

This is on the lastest version FreeBSD 13.

-- 
Ron Wills
https://www.digitalcombine.ca
 Open Source & The Command Line