Is it possible to definitely lose bitcoins? The maximum number of Bitcoin is 21 000 000.
However this number of Bitcoin may never exist.

Supposing someone have 5 BTC in their wallet. In order to protect them, they encrypted it and then forgot the passphrase, or die (sorry, this example is sad).
In this case, the real maximum number of usable Bitcoin would be 20 999 995...

Is that understanding correct? What do you think about this?
Yes it's possible to lose bitcoins forever.  There are even ways to "provably destroy" them by sending them to an address that can't possibly have a private key.