On call incidents shouldn't be routine. A rotation spreads responsibility across the team. Ultimately, someone has to be responsible for incidents and it aligns incentives best for it to be the engineers who maintain the code.
I recommend one per team. You want somebody who is knowledgeable on the specifics of any part of the system that could break. I don't think having more than 1 is a requirement though, if we're talking about small companies. As long as there is a "break glass in case of emergency" way to push to production that is auditable.
