Is the number of relief pilots regulated?
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Europe: EASA welcomes new flight time limitations rules (older article, possibly out of date)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.