Yeah, unfortunately, there doesn't seem to be consensus on the definition of a peak. Most definitions use some threshold of prominence, but with a mountain as massive as Denali, you will end up with redundant peaks for all but the largest thresholds.
The separation between Denali's southern and northern peaks is apparent
here. The north peak is about two kilometers north of the main summit and has a prominence of 402 meters.
I used
Wikipedia's ranking, which has a requirement of 300 foot prominence.
Subjectively, I would define Denali's North summit to not be a separate mountain for the ranking of the tallest peaks in the US (I guess choosing the prominence threshold to be > 402 meters), which would make Foraker the third tallest mountain, after Denali and Mt. Saint Elias.