a little apart from your question, in general and in theory bitcoin quantity can't go down, because its fixed 21 000 000 thats it
in reality, quantity goes down all the time, because some people loosing access to their bitcoins, other passing away without giving their private keys to ancestors, bitcoin are sent to addresses that noone own, so can't be retrieved etc.
So real quantity of bitcoin is not 21 000 000, actually is around maximum 17 000 000, 4 000 000 lost already