Is this a testable question how many moons does Jupiter have why or why not

No, there is only 1 distinct answer to the problem. rather have a question that can be measures through time like full moon cycles

Lately, this question has had a changing answer! For many years, we knew of 16 moons for the largest planet in our solar system. In the last few years, about two dozen new moons have been announced by astronomers I. However, being announced does not mean that these become "official" moons of the planet. To be recognized by the scientific community as a new moon involves a process which may take several years - and some announced objects never get recognized at all! Let's explain.

When a scientist makes observations which indicate a new moon, they submit their data and analysis to the scientific community.