At how many places on Earth are measurements made for earthquakes?

Answer

On the attached map of Belgium (from the Royal Observatory of Belgium) you can already count how many seismic stations are present in and around Belgium (see blue triangles). That’s about 30 already! And that for a small country with not too great a risk of earthquakes.
You can already imagine how many seismic stations there are in seismically active areas.
In short: there are seismic stations almost everywhere in the world. No earthquake will go unnoticed.