Have records ever been compiled for how many pilgrims return for additional Caminos?
Nope. No one even knows how many pilgrims walk just once. There are statistics for those who register at the Pilgrim's bureau in St Jean pied de la Porte and there are records for those that arrive in Santiago and request a Compostella from the Pilgrims Office there. As not all Pilgrims start in StJdP and neither do all who arrive in SdC claim a Compostella all the numbers, though interesting, are only an arbitrary reflection of actual Pilgrim numbers. In the absence of any central Camino "authority" or registration system (thank the Gods) no one will ever know.
I have no doubt that if anyone could access the Spanish Governments records of who stayed where on each and every given night of a year or two then there are some fairly good quality algorithms that could compute how many pilgrims were on any given camino on any given night though the accuracy would probably be no better than ><95%. But you would need at least the last 20 years of data to get any sort of grip on the serial repeaters.
There have been a few "surveys" over the years that have asked questions framed around "how many caminos?" but I've not seen any published results.