There is a sweet spot for how many people should be on a team. That number depends entirely on a nature of tasks. I would even go as far as to say - in a good environment up to that sweet spot, productivity gain is linear.
And yes, managers do expect close to linear productivity gain past that spot. Managers, that are a little smarter, start thinking about how spotify did squad (without actually knowing anything about it).
The term "Two pizza team" comes from Amazon and describes a team size such that it is the number of people that can be fed by two pizzas. The reality is that the term is not only a reference to team size but, rather, underscores the concept of "Accountability".
What I was talking about is something like this: SRE team has 1 member and productivity of 1, that person is overworked and if vacation is taken productivity goes to 0.
Hire second person, after onboarding productivity goes to about 2 (i.e. linear scale). Hire third person, productivity will probably be a little under 3 because now they need to spend time to be on the same page. Vacation is still shaky because with such small team, knowledge will be 100% siloed due to outside performance expectations ("hey you worked on X last time, I'm going to assign this ticket to you" repeated many times).
Eventually team grow to a point where they can handle all work load, share knowledge between each other and take vacations without fear (btw if you fear taking vacations - don't, it's not your fault if team can't handle without you).
You can think of "work load + process" as a data structure. If work load is bog and process requires a lot of synchronizations (every meeting is essentially a Mutex for the entire team) - you won't get linear productivity increase, instead you will increase lock contention.
And yes, managers do expect close to linear productivity gain past that spot. Managers, that are a little smarter, start thinking about how spotify did squad (without actually knowing anything about it).