For those of you who are wondering which one is the Korean Visa number in the visa, it’s the one on the upper right corner with the PH, and not the one in red characters that starts with 사.
Since I needed the Visa number for my Taiwan Visa-free entry/travel certificate application, I had to be sure which one it is. I looked it up online but not a sure answer came up.
Then I saw this photo on VisitKorea page.
I tried google-translating the Korean characters in no. 1, and voila! It says Visa number.
UPDATE: I also emailed the Korean embassy to confirm and this is their response.
The visa number in the Taiwan Travel Certificate..
Hope this helps!