How can I filter how many questions have been answered?

This would be step-by-step, so how long it takes to write depends on how many trials you have, but you could create one extra embedded data field per question/trial and then branch within the survey flow after each question so that: if the response to previous question is empty, ed1 = 0; else ed1 = 1. And then add up ed1 ... edN in the survey flow using something like ${e://Field/ed1}+${e://Field/ed2}+${e://Field/ed3}