The standard way of measuring accident rates is to consider the number of accidents (or fatalities) per x number of flying hours, so the size of the fleet is irrelevent.
Whilst there no doubt are many more twins about, I think most people would agree that there are enough C208 / TBM / PC12's etc. floating around now that they represent a pretty sizeable fleet in themselves.
Using the NTSB's figures from 1998 - 2007, single engine turboprops had one-third of the engine-related fatal accident rate of multi-engine turboprops.
That is a FACT.