Part of the problem with thresholds on mite counts is none that I have seen take into account the size of the colony, the time of year, the amount of brood etc. The number of mites that is normal in the spring is much smaller than the number that is normal in the fall when brood rearing ceases and all the mites are phoretic.
The typical numbers quoted, however, are 50 or so mites in a 24 hour natural drop being the economic threshold.