Have you looked at Gnu's "parallel" program??

It can take a script and run it 'N' times in parallel for multiple inputs.

There's also variant called 'sem' which uses semaphores to control the number of available "processes", but

'ls' -1|parallel -j6 du -sh
[download]

would run 6 du jobs at a time on your input.

Of great interest if you really want to figure out how to do such things -- parallel is written in perl!