Is the number of relief pilots regulated? This article hints that the number of relief pilots isn't mandated. Wouldn't requiring more relief pilots for longer flights be safer and ethical? I personally prefer airlines that overwork their pilots less!

I'm asking more about North American and European airlines, not just Asia. 


  As part of the agreement ending the strike, CAL [China Airline] pilots promised not to strike for three and a half years and the airline increased the number of pilots rostered for long haul flights. CAL now rosters three pilots on flights over eight hours, up from two pilots previously, and four pilots on flights over 12 hours, up from three pilots previously.
Pilot (and other air crew) rest time is an important factor in aviation safety. I'm not sure where you picked up that it "isn't mandated", but that is quite false. For example:


United States: 14 CFR § 91.1059 - Flight time limitations and rest requirements: One or two pilot crews.
Europe: EASA welcomes new flight time limitations rules (older article, possibly out of date)


Other aviation authorities around the world have their own regulations on how much pilots can work at once. These limits are very strongly respected and enforced.

I am not familiar with the regulations in the Asian countries referenced in the article you linked to, but I would be very surprised if they did not have similar regulations.