1) First of all? This is all conjecture. Nobody KNOWS anything. We can attempt to base our conjecture on what we do know about other large omnivores and great Apes.
How many? As low as 50 or maybe less.
https://www.conservationnw.org/our-work/wildlife/grizzly-bears-northeast/
How big of an area? Not big. Maybe for the US portion a 100 miles x 100 miles. Albeit steep dense mountainous terrain that includes the Salmo Priest wilderness.
https://www.mountainproject.com/area/106986809/selkirk-mountains
2) Read my thread Twist posted the link too. Being an Ape? They are an omnivore. Being a bipedal Hominid? They could be much more adapted to hunting than a quadraped Ape. Its theorized that human ancestors began working with tools when they stood up and freed up their hands.
3) They would have to migratory. Without farming crops or having domesticated flocks they could not adopt permanent settlements. Not unlike primitive tribes of humans have done in the past.
It does seem unlikely that in the Jet age, we could find ourselves not alone as the only bipedal Ape left on planet Earth. But there has been many archeological finds recently that point to the fact that there is alot we do not know about the human tree and even just within the last 10,000 years what was going on.
https://www.livescience.com/29100-homo-floresiensis-hobbit-facts.html
https://www.newscientist.com/article/2128834-homo-naledi-is-only-250000-years-old-heres-why-that-matters/
https://www.livescience.com/62036-modern-humans-interbred-neanderthals-denisovans.html
https://news.nationalgeographic.com/2017/04/mastodons-americas-peopling-migrations-archaeology-science/