How many to keep the lights on?  

About 4 years ago I was told 11-13 was the minimum needed for an average AMC base to break even.  Every place is different though, payer mix and types of flight vary too.  You might be doing 30 a month but if those jobs dont pay then thats worse than 20 flight but all pay well.  Hospital vs community bases have different expectations and needs.  A hospital might be making money on having those critical patients brought to them where they do well and dont worry about the billing for the actual ride there.  Lots of variables.