How many experts needed to validate any instrument? and if the instrument is widly used and the litraure was support the instrument, still we need to validate it through group panel of expertes?
It depends on what is expected from the instrument scientists and engineers you assemble in the review team! For instance I'm working as a member of a science instrument definition team for a space mission concept where our mandate is to write a detailed report of various instrument concepts that can be on the payload! We have a team of 12 people that meet once a week for one month to produce this report!
if you are reviewing validity of instruments used in a clinical setting then I would definitely have in your team, people that manufactured the instrument,, people who carried out clinical trials, users of the instrument, electronic and mechanical engineers and scientists that are specialists in the instrument function!