So at the link that ianhawes helpfully provided, the US government describes the terrorist watch list as "[having] information on people reasonably suspected to be involved in terrorism (or related activities)."

I don't think we knew how many people were on it before. But if it's a strict superset of the no-fly list, then we now know it's at least 1.5 million people.

I guess the only odd thing is that a lot of people seem to believe the USA government's line that more than 1.5 million people are "reasonably suspected to be involved in terrorism or related activities", when there are maybe a few dozen terrorist attacks a year. ("reasonably"! "related activities" huh?).