It depends on the use of the word 'play'. In my case it's probably somewhere between none and one.
At a Christmas party some years ago, there was a piano. Several people got up and played mostly carols to which we all sang along. After my son got on the piano and played a few Christmas hits, e.g. Last Christmas, I said 'I didn't realise you could play the piano that well'. He replied 'I can't play the piano at all, but I understand how music works'.
So, maybe there's a difference between actually knowing how to play an instrument and being able to get a tune out of it.