You earn 2,000 miles' worth of upgrades when you fly 10,000 miles. That's where the basic 20 percent figure is from. As a practical matter, you get less than that (as gemac correctly pointed out) because you need, for example, six "stickers" (3,000 miles worth) to upgrade 2,611 miles BOS-LAX. The remaining 389 miles' worth of potential upgrading are wasted.

The post which you quoted reflects these things:

1. Overseas AA flights earn sticker credit, but can't be upgraded with them.

2. Partner miles earn sticker credit (subject to fare class and earning percentage rules) but cannot be upgraded with them.

3. RJ flights earn sticker credit but, because RJs have no F cabin, also can't be upgraded.

Bottom line for these three: you can only use stickers on mainline AA flights within North America, but you earn credit toward them other ways as well. Anyone who flies much in any of these three categories will have enough stickers for well over 20 percent of his/her mainline domestic AA travel.

4. Finally, one can't upgrade if others get all the upgrade seats first. This rarely happens to EXPs, but often enough to PLTs on elite-heavy routes and quite often to Golds. Again you earn credit toward more stickers, but don't use any.

The net effect is that, depending on flight patterns as regards overseas, partner and RJ flights, PLTs and especially Golds may earn all the stickers they can use or pretty close to it.