Where is finger number 1 in music?

It depends on whether you play the piano or whether you play the violin.
If you play the piano, your thumb is finger number one.
But if violin is your instrument, then your pointer finger is finger number one.
Talk about confusing if you play both!
Since I play the piano, I advocate the thumb as finger number one.  Most adults hold up their pointer fingers when I ask them for their first finger.  Interesting!