What is the mean number of mating cycles of a FMC connector?
These numbers are not the maximum number of mating cycles.  They are the manufacturer's indication of the minimum number of mating cycles the connector will withstand and still meet the spec.  Most connectors will take more than that, a small few will probably fail before that number of cycles. Things that can affect the number of cycles stated are:


The quality of materials used in making the connector. Similar looking connectors might use different plastics, for example.
The intended use.  Some connectors are expected to be put together once, during assembly of the device they are in, and never touched again.  Others may be taken apart regularly.  This influences the choice of materials above.
The company giving the rating. A large, well known company may be conservative, and quote 500 when they expect the average to be 1000, whereas a cheap no-name manufacturer may state 6000 in the full knowledge that few connectors will survive that long.


So if you want to know the number of mating cycles for a particular connector, the only way to find out is to look at the datasheet, and multiply the number given by some number between 0 and 2 depending on how much you trust the manufacturer. No datasheet? Ask the person/company who supplied it.