My plane to Manila international airport arrives on 20:30 and I would like to reach Calamba to stay for night. Will I find any bus for this trip?
There are no direct buses from the airport, you'll need to go to one of the major bus terminals. It seems like JAM Liner are the only bus company that will be running that route at that time. Rome2Rio suggests other options as well but they're more complex I can't really confirm they exist.
There should be buses that will take you from the airport to somewhere near the terminal you want, but different companies have different terminals so it may not be direct.
All of this depends on your willingness to try and navigate Manila. If it's your first time to the city I'd strongly recommend against trying to do this all on buses unless you're used to navigating chaotic public transport systems and are prepared to end up spending a lot of time waiting.
If you're determined to save money I'd recommend taking a taxi to the bus terminal and getting the bus from there.
But consider either just taking a taxi directly (~$40, you'll need local money, get a metered taxi). Alternatively ask your hotel, if you have one already, or a local travel agent to arrange transport from the airport.