As-salaamu alaykum wa rahmatullah,
According to Harvard's Pluralism study, the range is from 2-6 million. Allahu A'lim.
And Faizah's points on statistics are important, especially when considering groups that are not found homogenously around the country (like Jews, Muslims tend to concentrate in metropolitan areas). I would guess that's taken into consideration, but I don't know, haven't looked into it. And I've always thought we must be behind Jews in #, considering American history, but there are so many social reasons why that may be changing, considering the rate of change. Hence, the wide amount of conflicting data.
The only concern I have with this study (besides not the most accurate data in general), is the finding on how many people seem to "leave" their religion...scary. Unfortunately, I see it more and more often, esp. in 2nd and 3rd generation Muslim families. I know someone (2nd generation) who has never been taught a lick of Islam. Literally, her non-Muslim friends know more about it than she does. So sad.