We already know from millennia of history that the human population can be maintained just fine at pre-industrial technology and population levels. Sure crop failures happened, but they don't happen everywhere at the same time. Famines were historically very localized things.
And if we're talking complete climate collapse? In that scenario, everything goes out the window. If that's the scenario you want to plan for, then we should be doing everything we can to shrink our numbers as fast as humanly possible.
Humans are by far the most resilient species on this planet. We've learned to live everywhere from burning arid deserts to frozen arctic wastes. We will be the last species on this planet aside from some bacteria that go extinct.