Answers a simple question:
“How many developers have to leave your project for it to come to a complete stop?”
Bus factor value should be equal to a number of developers on your team. The bigger team you have, the safer it is for your project.
Low bus factor in large team
If bus factor value is significantly lower than number of developers on your team, there are no quick solutions.
You can gradually improve it though practices such as peer-review, cross-training and diversifying tasks of your developers.
Low bus factor in small team
Low bus factor value in team of 1-2 developers might not be critical.
To make it safer, besides growing the size of your team you can make sure your project is well documented.