I wasn't aware they had a number, or if they did that it mattered. Just plug into any of them. If you have 3 DP monitors, plug into all 3 and rearrange them in Windows.
Easiest way to tell is plug in multiple monitors and see which one your BIOS/POST displays on. On Quadro's they do number the ports (really hard to tell, the engraving is tiny, but they are there) but I don't recall my 3080 having those ports marked. If they did, Like criccio said, I don't care, I let Windows handle it.
Judging by the fact my BIOS/POST comes on on my secondary display in the middle, and the primary is plugged in next to the HDMI, I'd say the right DP (if you're looking at the back of your computer) is Display 1.