Ray, for the 2017 event you can review all the information here in great detail:
www.mercedesbenzchallenge.1000 miglia.it
According to the printed regulations up to 60 Mercedes vehicles can be registered of which 35 places are reserved for historic vehicles (prior to 31/12/1990). This year there were 27 vehicles shown on the official starting list, only two of which were "historic." These are the two pictured in my earlier post.
I may be wrong but I suspect that participation cost may be the reason for the small number. The base cost is about €9500 per car entrance fee which includes some hotels and meals for crew but there are many other costs to add (support crew cost, travel, possibly shipping cost, etc., etc.).
How much are you willing to pay for four days of extreme enjoyment?