Can't imagine that there's a reliable source . There's no counting and lots simply flit over, unannounced. If there's an average 100 boats per week over the whole year, that's 5000 (obviously!), praps this at top end, praps not...
Consider too the number of YCs and owners associations that rally across, plus the race fleets for things like the RORC Deauville, or the JOG Cherbourg.