There were 3500 riders last year, about 2800 the year before. I would expect with the 2 distances they have room for more riders. 5600 would mean the ride has doubled in size in 3 years. That's huge - and a huge impact on the logistics.
I personally can't see them going much over 3500, maybe 4000 tops. When you see the field of tents a Mohawk you'll see why. With 1500 tents for 3000 people they were close to out of room and the first year (because of a bit of early morning rain) they weren't even close to having all the tents set up in time. With 5600 riders that would mean over 2500 tents, I just can't see it happening.
I think one of the reasons this ride has been so amazing is that they've kept the total number reasonable. If you've done the Heart and Stroke DVP ride with 10,000+ it's just mayhem and not much fun as there are just too many people of different abilities (slow people not paying attention and people who think it's a race, recipe for disaster with those numbers). So hopefully the 5600 number is just a rumor and not fact, that many people I think they'd be biting off more than they can chew.
Just my 2 cents and no disrespect meant to organizers either, it's been amazing to date, but again because there has been a cap.