In a world where some large fraction of the working-age population is employed in factories (most of those in automotive), maybe not so many should be musicians. In a world where we've shipped all those other blue collar jobs to Asia, every industry sub-sector that becomes unviable is a disaster. So asking "how many x should there be" sort of marks you as clueless or even callous. The answer is as many as there can possibly be, plus a few extra.