Great questions James. I've been a wildlife biologist for 30 years. The one thing that can be stated with a very high degree of confidence is that nature takes perverse pleasure in making fools of human understanding. With that in mind, every species archetype requires a specific number of individuals for a healthy population. The more complex the organism, the greater number of individuals are needed to avoid species collapse at the genetic level. The smaller the gene pool, the harder long-term survival becomes. There are a large number of factors that have to be considered when calculating the minimum viable population for a species. In humans, the MVP can be as low as 15 individuals or as high as 4000 individuals depending on the variables used, conditions considered and potential morbidity and fecundity rates. We simply don't have enough information regarding bigfoot to make any realistic guesses as to whether the species is growing, stable or in collapse.
What they eat will depend on what they are and that hasn't been answered yet. There is a lot of anecdotal information that indicates it's a largish mammal. Popular conjecture is that it is a primate. A strict vegetarian mammal requires a long gut to break down the plant materials consumed. In primates, this normally results in a pot belly physique. Lowland Gorillas are a good example. As protein intake increases, the body shape changes and gets slimmer in the middle. Most reports indicate bigfoot has a high protein diet based on general body description. Another indication of a high protein diet is intelligence. Bigfoot is reported to be very intelligent. If true, then protein is likely present in their diet. There are a number of primate studies that conclude that primates with diets containing 20-50% protein exhibit higher intelligence than strict vegetarian primates. Lowland Gorillas vs. Chimpanzees. You also have to consider brain size. Elephants are fairly intelligent for being herbivores, but their brains are 11-13 pounds compared to humans that have 3 pound brains. However, these musings are all guesses based on what "should" happen. It's inductive reasoning and suffers a few ad ignorantiam fallacies, but fun to consider.
That brings us to how much a bigfoot eats. That depends on diet, metabolism, activity, and standard environmental conditions. That requires subjects for study and they seem to be difficult to locate reliably. Until someone figures out how to observe bigfoot for days at a time without them knowing, speculation will be the only information you will find. The same goes for where they sleep. It might be in caves, ground nests, up in trees, in abandoned mines, abandoned structures, or gullies. There is no consensus on this subject, but bigfoot seems to be as opportunistic about its sleeping arrangements. It if is as intelligent as the pundits believe, then it will take whatever the most advantageous accommodations are at the time.