As @pyramids and @john-devor explained, as it stands today, the bitcoin mining rules will not make up for any lost bitcoins. So in theory the total amount of bitcoins in circulation will never reach the full amount initially defined by the protocol.
If you think about it, it's very difficult to identify "lost" coins. Just because a wallet is not touched in years, it doesn't necessarily mean the private key was lost (as likely as it may appear).
Having said that, Bitcoin is only sofware, and that software can be modified in the future.
If somewhere down the line people find that there aren't enough bitcoins to serve the global economy (remember that most people will not be trading BTCs but mBTCs or uBTCs, etc) nothing is stopping the community and developers from agreeing on a new ceiling and implementing that in a future version of all bitcoin clients, effectively making more coins available for mining.
It would be a tricky change from a socio-economic perspective, because it would have a significant impact on the value of the existing coins, but it is possible.