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.

I know this "donation to everyone" idea is from Satoshi Nakamoto's own commentary (or at least that's what I recall), but I don't see it as positive.
If it's true that around a third of all coins are basically lost and gone forever, that hurts far more people than it helps.

The few who bought in early might gain in value, but everyone else in the world loses out. That's a comparative handful of people having coins worth slightly more in exchange for the entire world facing a situation of higher prices and less likelihood to want to transact in BTC.

Why does "everyone else in the world" lose out?
1) There will always be enough BTC, since you can add more zeros and someday buy an awesome new car for 0.0000000000001776 BTC.
2) The eventual race to own "at least one whole Bitcoin" will drive prices to levels that might (today) seem very insane.

Edit:
3) High prices do not drive people away, just wait until you see your first Bull market, Newbie. Cheesy