The proper, though not easy, thing to do is have some primary sources of your materials and secondary sources to handle surges and supply issues with your primes. You may still have a production slowdown, but hopefully not a shutdown. And like with backup, if you don't use them you don't have them. You have to place orders with all sources, and use materials from all sources to ensure that they are in fact up to snuff. It'd suck to buy your RAM from Foo until a flood, and then find that BAR's RAM doesn't actually work (or doesn't work with your motherboards).
But, having a single source is more efficient than multiple sources---you can integrate tighter with their ordering system, packaging, product quirks. Until it stops working.
Having all of your sources in one country is more efficient. Until it doesn't work.
Managing all of your materials just-in-time is more efficient. Until a backhoe hits the gas main in the street outside and you can no longer get trucks into your factory and have to shut down the line for the duration.
A company with a weak (but not completely idiotic) supply chain will have a significant margin over a company paying extra for a strong supply chain. Until something goes wrong.
You do not have to live for ever, so surviving a extinction level ecvent is more than the "right" level.
Being so brittle that the failure of one delivery kills you is less than the "right" level.
How long do you want to live? And when you die, how is it going to happen?
It is pleasing to see firms go out of business in a orderly fashion, such that they have the resources to shut up the shop properly, and every one in the business gets "onto the life-rafts". But it is more common to see businesses go down in a screaming heap with huge debts and big messes for others to clean up.
When I was at business school I was taught that a firm should have the correct amount of debt such that it maximised the tax shield of interest payments and had no fat that a hostile acquirer could use to do a hostile takeover.
That was in 2007. I am quite sure the course teaches something different now!
"Sufficient" is dependent on a lot of factors, though. No one can tell you a definitive answer without knowing your system (your suppliers, your customers, your rates of production, your supply of capital to withstand a drop in production).
Buying in bulk is cool. If you buy a big package of paper towels, it's cheaper and you don't have to worry about running out. The fact that you have a cabinet full of paper towels isn't a big deal. But suppose you find a really great deal on a semi-trailer load of towels and stock up. Now you have your guest bedroom full of paper towels. The next week, your cousin Edgar's house burns down; you'd like to offer him and his wife a room to stay in temporarily, but you have all this paper in the way. You have lost some flexibility.
A bigger problem is, say, corporate supply chains. With just-in-time supply, you don't have to store inputs and can focus on producing outputs; it's very efficient. But then there's a pandemic or a fire in a factory somewhere, and the supply chain falls apart. Now your outputs are perhaps in greater demand, but you can't take advantage because you have no inputs. You're out of business for the duration. You can't flexibly respond.