I think a decent rule of thumb might be to have three people with really deep holistic expertise. If there's so much context that it's hard for people to have a

I think a decent rule of thumb might be to have three people with really deep holistic expertise. If there's so much context that it's hard for people to have and maintain that depth, then the organization is successful enough that it can afford to pay a premium for a few of them. And when one of them moves up or out, you have two more to pass on the knowledge to a new third. Maybe two or four or five would be fine too, but three seems to me like it might be about the optimum. But it's very risky to have either zero or one person who knows a system from top to bottom.