To be honest, I don't have a good answer.
Indeed, I don't have the friend network since I stopped forcing myself to be social. Most people don't call again if you don't answer their calls for a few months [1].
I too witnessed people calling their friends for help in hard situations (like car broke down at 1am) and wondered who I could call if that happened to me.
I have a family and extended family who are very supportive. That is my social safety net, and they would help in any serious issue like money, car broke down at 1am, etc. This is a part of our culture where I live (It's not the US.) If you don't have that, it gets trickier.
For example, if you live in a different city than your family (which is a historically new phenomenon) you can't rely on them for car breakdown at 1 am situation.
Setting aside family, I think, for life-and-death issues (e.g. cancer) even people who know you at a superficial level (e.g. classmates, colleagues) would be willing to help, at least where I live.
For middle sized issues (car breakdown at 1am) I could call exactly 1 friend in my hometown, and 1 another in the city I study. They are close friends who like me as the person I am. They are content with our low-effort low-contact friendship. If you want advice, I think if you meet enough people (that's the hard part ofc) you'll eventually find one of these. Funnily one of these friends is extremely social, the sort of person you'd expect to greatly succeed in politics.
For smaller matters (e.g. an assignment in college) I have to admit I refrained from asking people for most anything ever since I stopped being forcibly social. This did lead to some (small) losses over time.
I live by the principle of "never ask any more than you gave to a person". Though it's not uncommon to see NT people who barely know each other confidently ask for small favors, offering nothing in return, simply because they don't have the social anxiety that I do.
1: I know it's rude. But iykyk the anxiety.