I can't seem to find any sources for this. I know that in 1606 Samuel de Champlain, exploring the Massachusetts coast as far south as Cape Cod, was dissuaded from establishing a settlement by the size of the local populations and advocated against French colonisation of the area for that reason. When the epidemic of 1616-1619 swept through it killed anywhere from 30-90% of the Native American populations, the Wampanoags being the hardest hit. Wikipedia (not the most reliable of sources) says that at the start of King Philip's War in 1675 there were only about 10,000 Native Americans left in the area affected by the conflict (basically the land east of the Connecticut River and south of present-day Massachusetts' northern border). I'm having trouble finding any population numbers for the years prior to that though. Can anyone help?
Charles Mann, in “1491” estimates that the Wampanoag nation had 20,000 people in 1617, and only 1,000 survivors in 1619 (one year before the first European settlement in their territory at Plymouth Plantation), after an epidemic disease spread from French shipwreck survivors on Cape Cod.
I don't know where Charles Mann got these numbers from, nor how reliable they are.
Daniel Gookin, an actual colonist, said
[the Wampanoags] could raise, as the most credible and ancient Indians affirm, about three thousand men"
If Massasoit could raise 3,000 adult men, there would have been about 12,000~15,000 Wampanoags in total.
Assuming that the Massachusetts, Wampanoags, Nausets, Narragansets, Nipmucks, Pequots, Niantics, and Mohegans all had around the same population, which probably isn't true, there would have been 96,000~120,000 people in the region. But of course, the Nausets or Niantics would have had a much smaller population, meaning that this estimate isn't perfectly reliable.