I left a comment on this question suggesting to try a cross-database query to get a reliable statistic (I did mention that it might run for too long but that is not the point).
After seeing nicael's answer I realized I didn't recall seeing cs50 or edx-cs169-1x among the SEDE dabases. And that makes my comment misleading.
Those sites seem to be different with respect to the process they followed and have slightly different/added functionality. They are, however, still part of the SE network.
What is the reason those sites are not available in SEDE?
If this is a simple oversight, can those be added so I could in theory answer this question with one of my nifty sql scripts?