Impossible to know... but when they were nearly worth nothing in the start, lot of them were lost for sure.
Yes, we can't be sure of the exact number of lost bitcoins. But:
1. It is not important.
2. It is actually a good thing that bitcoins are lost.
3. Lost coins only make everyone else's coins worth slightly more. Think of it as a donation to everyone.
There must be a market for bitcoin treasure hunters?! It's basically hacking, but authorised by someone who realises that they had 100k Bitcoin on a computer that they threw away, or lost the passwords for!
There was a story during the last boom about someon looking threw a tip to find his old PC hard drive because he has 1000BTC on it!
30% lost currency is crazy if you think about it, so really on 15mill will ever be usable