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The number tracked is the number of items that you have started studying and are caught up on reviews for; that is, they are not currently due. This is the number that represents real progress; if you only track the number you've started, then when you fall behind on reviews your number stays high but you might have actually forgotten everything and not made any true progress. This number is also useful when you're trying to work down a backlog of due items, since it's measuring the increase in caught up cards.
It uses the highest number it gets for the day, so if you make sure you're on track, new items coming due will not cause you to derail later in the day.