In typical Harley fashion they don't seem to list any real specifications in the manuals for the system, at least none I could find, so your guess is about as good as anyone's guess at this point. I would suggest calling H-D Customer Service, but I would hate to get your hopes up. Maybe someone in the aftermarket will figure it out and let everyone know; Harley can't be expected to know the details and specs of all the stuff they buy from vendors and sell to
their customers.

Btw, the real storage hog for any GPS device I'm familiar with is the actual map data, not the custom routes. Route files are relatively small and are usually measured in KB, not MB. The potential problem won't be how many custom routes you can save, but how long will the system work before the new updated maps become too big to load to the device. That's a common problem with stand alone GPS units over time, and it is what makes it necessary to throw away perfectly good old devices and replace them with new devices. We used to call this planned obsolescence back in the day. It's a pain to have to throw away a perfectly good $100 GPS unit and replace it with a new bigger $200 GPS unit, but when you're talking about a built in system that carries a price tag in the thousands of dollars it goes from being a pain to being a deal breaker for someone like me. I've never sprung for the factory built in GPS in any vehicle I've ever bought for that very reason.
Jerry