Yup. If you look at the math, there's two episodes for each of the other nine worlds. That brings us to 18, plus the premiere episodes. Argent is right. We'll be at Episode 19 by the end of the traveling, which leaves us with eleven episodes to finish it all up.
I actually don't want Decade to have original villains. I'm really hoping what it turns out to be is an army of head villains from the original nine rider worlds conspiring together to destroy all the riders. That would be awesome.