Suppose you have 1btc and you forgot your wallet password and even your private key if your wallet so can you access back to your account. Obviously your money will get locked and never in your lifetime you could use that amount and that amount will be there still in your wallet and in Byron network but it's not usable.
Actually bitcoins quantity can go down. Well it has happened before. There are many bitcoins that has been stored in hardwares and other storage and it was damaged, lost was thrown out and that has caused the number of bitcoin to decrease. The problem with hardwares and saving bitcoin on desktop wallet or in a usb if anything happens like damage to computer and hard drive may cause losing your bitcoins and it will affect the total bitcoins in circulation.