Well, SNS may not be about cell counts, but you need to be in the right ballpark, and 20 is not close enough to 200. I am not an expert, but my understanding is that you should step up cell counts by about a factor of 10 each time. If you want to step up your population you could add your yeast to 1/10 of a liter of wort, let it ferment out and flocculate, then drain off the liquid and pitch the remaining slurry into a 1 liter SNS starter. That should get you enough healthy yeast.