How much condensation is okay on storm windows?
How much moisture is ok depends on the window frame material and the material that condensation may drip or seep onto. The primary issue is rot. If your storm windows are aluminum or vinyl, some condensation will not likely cause damage. Wood windows will see degradation of their finish and ultimately wood decay. 

The only way to completely prevent condensation may be to increase ventilation between the storm windows and the inner windows, but that obviously decreases their thermal effectiveness. 

Your best bet, if decay is a concern, might be to forego the old-fashioned storm windows and use plastic sheeting on the inside of your windows. In my experience, you may still see some condensation on your windows in this case, but only as ice when it's extremely cold.