Already we have limited supply of Bitcoins i.e. there will be 21 million bitcoin that will be ever generated. There is no clear figure that tells how many Bitcoins are there that can't be accessed since dormant wallets are getting active very often. There are many cases in early days of bitcoin, when people used to send bitcoins to each other for fun and for free. Lots of such coins can't be accessed now because the owner lost the seed due to his casual attitude towards bitcoin.