So one of you is an Eagle Scout? That means you have survival training and stuff, right? If so, how much food could foraging possibly provide? For a few dozen people I could see it letting them live for a short while, but I find it very hard to believe some wild mushrooms and berries are able to feed more than that for longer than a day!
So, a quick caveat, while I do have wilderness survival, foraging is one of the topics I remember the least about. Some of this is just practice. My orienting skills are still fairly sharp, but I actually use those.
You’re coming to this from the wrong perspective. It’s entirely possible for a couple dozen survivors to live off the land indefinitely, but it’s going to depend on them working together, and their food supply’s going to be a lot more diverse than just some mushrooms and berries.
Foraging mushrooms is something with a very low margin for error, and something I’d personally avoid. Identifying and distinguishing between poisonous and safe mushrooms requires you have a pretty solid grasp of the local fungi. Screw up and you can kill everyone.
Berries are a similar story, though it is easier there to test and determine if they’re toxic in the field. This involves exposing yourself to trace amounts of the juices and carefully checking to see if you have any toxic reaction to it.
I do remember how to set up traps for small game, and how to obtain meat via hunting. It’s an entirely different skillset, but it will keep people fed. I also remember the methods for water purification, so, again, that’s a necessary step for keeping survivors alive. I don’t fish, but that’s also another food source that can’t be overlooked, if you’re trying to keep people breathing.
Also, worth remembering, your survivors need to be able to cook their food. First, it kills many potential pathogens, so your survivors are less likely to get sick from what they’re eating. This also improves your body’s ability to convert that food into energy, making the food (effectively) more nutritious.
Okay, so, let’s step back from this for a second. The real question here is how long can a group of survivors last, when one of the people in the group has survival training? The answer depends on their surroundings.
The community needs three resources, in this order of priority: Water: Without safe drinking water they will die, soon. Food: As with water, this will kill them, but it will take longer. Their ability to function will be impaired over time if this resource isn’t there. Shelter: This is critical for a number of less immediate reasons. Your survivors need to be able to avoid the worst of the weather, and a space where they can safely recover from the foraging or hunting. They’ll also need a cooking space, which is part of the shelter topic. This lets them turn the water they find into safe drinking water, and it allows them to convert the animals they can find into a food source. On a long enough timescale, the shelter will transition from an adhoc setup to a permanent structure, but that’s down the line away.
Foraging in temperate environments, particularly lightly wooded plains is pretty easy. The more hostile the environment, the harder it is to find food, and additional considerations start to filter in. For example, a group of survivors in a forest where the biggest threats are hostile wildlife, should be able to survive basically indefinitely, with a fairly solid protein diet.
When you’re looking at an alpine or tundra environment, food is less common, there’s still flora and fauna, but the edible plants are going to be less plentiful. There will be additional physical stress on the survivors because of the cold nights. There are a number of ways to help combat this, but cold nights are an issue for a wide variety of biomes.
If you’re looking at a scrub-land or desert environment, food will be there, but it’ll be harder to locate. You’ll also face greater issues with finding water, and extreme temperature shifts. In some environments this can even result in daytime temperatures that inherently dangerous, with nighttime lows. Finally, obtaining water becomes a serious consideration.
The upside with survival training is, most of it is easy to teach. So, if you have a group of survivors, and one of them has prior training, it’s very easy to teach other people what they need to know to start taking roles in keeping the group alive. This is also the critical part in, “how long can they last?” If they’re going to be operating in survival situations for long, it’s absolutely critical that the community start distributing the workload. Keeping two dozen people alive is too much work for one forager, but it’s an entirely reasonable goal for a group.
The equipment your survivors have with them will also affect how easy or difficult it will be. If they have some cooking gear, and small tools like hatchets and knives, that will make life significantly easier. If they don’t, they’ll need to improvise tools as they go. This makes life more difficult, but it is doable.
You’re right that one person foraging for berries and mushrooms won’t be able to feed a large group. The big part of survival training is understanding there’s a lot more in your environment, and how you can use that to stay alive.