This is a highly subjective question - is your production seasonal? Is the increase in production temporary and can you hire skilled temporary labor to cover the increase? I have worked in various manufacturing industries and never heard of a particular rule of thumb. I believe your best guidline will be a realistic sales forecast (salespeople are famous for "puffing") in concert with a production schedule. What you do not want to do is hire people and then have to lay them off in 6 months - make sure the headcount adds are reasonably permanent. Sorry I can't be more definitive....