The list includes everyone who wants either a first class upgrade, or a premium class upgrade, or both. (You can choose to opt out of either upgrade, so it's not necessarily true that a person has requested both.) This is why the list is very long.

First class upgrades are listed first, which is why premium class upgrades start with higher numbers. Your number really doesn't matter. What matters is how many people above you have not been upgraded yet.

In theory both kinds of upgrades are being processed simultaneously. Being upgraded to premium class DOES NOT affect your chances of being upgraded to first class.

Some people appear on the list even if they already have a premium class seat because they selected it in advance, probably at the time of booking, or perhaps they already paid for it. I believe this is a bug in the system that allows passengers to stay on the list even after they have a premium class seat. But remember, since this one big list is also being used for first class passengers, that's probably why they continue to appear.

Remember that because all upgrades are determined by status and fare class, those passengers who have to wait for a premium class upgrade probably also have low priority for a first class upgrade. Passengers who have already received a premium class upgrade will probably have higher priority for a first class upgrade. But it's not guaranteed and depends a lot on which fare you buy and how many seats are available on the plane.