What is a bus number?
How many people on your project can be hit by a bus before your project fails?
If it's one, that's a bad number.
Our bus number is 5 - that's how many programmers on our product development team could get hit by a bus before it affects our project.
A high bus number is your project's insurance policy
By estimating your bus number you can quantify the risks of your project while it's still in a vulnerable state of development. Growing your team of programmers increases your bus number, and because each person develops an adequate understanding of the overall project, work can continue seamlessly should something happen to one or more of your programmers.