How to schedule system calls with Python

[Jeremy]

I need to write a Python script that will call some command line
programs (using os.system).  I will have many such calls, but I want
to control when the calls are made.  I won't know in advance how long
each program will run and I don't want to have 10 programs running
when I only have one or two processors.  I want to run one at a time
(or two if I have two processors), wait until it's finished, and then
call the next one.

How can I use Python to schedule these commands?

Thanks,
Jeremy