Truck number is the minimum number of people who can mess up a project. The higher, the better.
For example, the truck number is 1 for a project, meaning that when one person got hit by a truck then the project cannot go on anymore.
If the example is too cruel, then we can also call it lucky number. For example, a person get big money by a lottery, he quit the company and the project cannot go on anymore. It means the lucky number is one.Now our company has one HR person. He is out of country more than 30 days. I need to talk to him about my Visa but I cannot process it; I'm wondering he might actually get a lottery.
One may think higher lucky number means less efficient, because of too much redundancy. Well, it may be true. Insurance is for reducing risk not to increase efficiency. And the risk may mess up your company or your life.
My company is small as a matter of fact. But I do not think it is as small as it can afford only one HR person. I do believe it is rather a matter of experience. People who don't have various experiences tend not to prepare for other risky situations.