Some estimates by experts claim that the number is between one and seven million Americans with a dual citizenship of some kind. My gut tells me that this number might be low. There are other estimates claiming that over 40 million Americans are eligible for dual citizenship. This number seems reasonable.
Going back to the number of Portuguese dual citizens, I will make an attempt to guesstimate. There are about 1.5 million people of Portuguese decent in the United States. Many are already Portuguese citizens who have emigrated recently. Another portion that were born in the United States are not eligible for dual citizenship via Jus sanguinis, or by blood, since that can only be traced back to grandparents and they might be too far removed to be recognized.
By just totally guessing and throwing caution to the wind, the total number of ethnic Portuguese eligible for Portuguese Citizenship could be about a quarter of the 1.5 million or 375,000.
Comment below to let me know what you think about this wild guess.