Edit - After writing all of the stuff below I noticed you are using a non-routable mapset. In that case, if you have the routing set to "off road" there should be no issues with route recalculation. What maps came preloaded in your GPS? Do you have the shonkymaps loaded to it too?
The number of points in your route is listed right there in the route properties, no need to count them...
The problem with using the "invisible via points" (I'm familiar with WinGDB3) with a Nuvi 500 is that there's no way to turn off the automatic off-route recalculation. With regular via points, if you go off your route it will just recalculate to the via point in your route you missed, then resume your route unchanged. If you use the invisible via points (also called "shaping points") if you go off route for any reason it is going to recalculate your route all the way to your end point. The only way to get it back will be to stop navigation, then restart your route. Select "no" when it asks if you want to be routed to the starting point of the route.
IMHO you need to see if you are using more than 250 via points per route and just reduce that number.