I have been thinking along the lines of Fi-Q's scenario, and all the way down to large areas of the world, or the whole world, just running out of the means to either produce or pay for modern conveniences. There is an enormous difference in how quickly the event takes to impact on humanity, the after effects on those left alive, and, in different countries, the numbers that would be left. Somebody posted late in the thread that people from other countries do not realise how big the US is. A lot of people do though. I farmed/ranched in Australia and its population (I am not going to do any checking so no exact figures here) of about 20 million is a lot less than the US. Close to 300m? The area of both countries is quite similar if you take the mainland States without Alaska. I know Australia has a lot of forested land too, but I think it will be less than the US and Australia might have more desert. I think I would prefer to be in Australia if it really came to the crunch.
So, we have to decide which event is likely to happen. I choose the second because that would be my choice between the two. Whichever happens, and we have to assume we personally will survive a sudden catastrophe, when to we begin to stockpile all these things we are going to need? Now, next year, 10 years time? Who is stockpiling now?
I am not stockpiling anything and have no intentions of doing so. Instead I am continuing with producing as much of our own food as reasonably possible, but so long as the supermarkets are open and I have money I buy other things I want either by choice or necessity. We make soap, but washing powder is a lot easier to use, I make wine but other people make it better, I like bananas and cannot grow them. We are on the grid but can get around that if it fails. I presently buy fuel but am working towards some self-sufficiency - methane, ethanol, olive oil biodiesel, even possibly grape brandy.
I have a family/friend group of 4 couples worked out. Me, an electronics/mechanical bloke, two scientists with a wide range of experience outside their own field, a female medical doctor, a female vet, and the other two females with long experience of most domestic crafts. Everybody is "countryfied" and collectively have knowledge and experience of a wide array of skills. If the catastrophe happens suddenly we cannot all get together, but a slow downhill spiral has already been discussed. The choice of abode for that scenario is here in Portugal.
If my wife and I are stranded on our own, then we already know we can survive for several months without contact with the outside world, but I wonder how many of the other posters have experienced such isolation? What is the longest time that any poster has spent isolated from contact with all other humans? A single person or couple is the numbers limit. Family groups can probably go on forever.