Trading volume. It was taught to me so long time ago and I have thought it so obviously true that I haven't questioned it ever. If you want sources, here is one example (TBH, I only read abstract) I quickly googled. To me, number of trades does not sound a reasonable measure for analyzing markets in macro level, even if for HFT it may be interesting, I know nothing about that.

https://core.ac.uk/download/pdf/4837179.pdf