As somewhat of a newbie to the intricacies of Bitcoin, I am curious about "lost coins."

Is there a way for us to know exactly how many lost coins there are?  Say coins not circulated in 5-10+ years?

If not, it seems reasonable to have those coins be able to be returned to circulation, somehow.

Also, say some something catastrophic happens, war, earthquake, tsunami, etc.  Hundreds, or even thousands of BTC users wallets are lost forever.  What then?  The coins are lost forever as well?  Will the rest of us know how many are left to circulate?