Isn't the ideal teams size just one? I do not mean that the one-person "teams" should not meet from time to time to agree on some common issues. But the ideal situation is that the work should be split in such a way that a single person has complete responsibility over an area and does not need to coordinate with someone else most of the time.
A team of one also cannot have multiple experts gathering to solve a particular problem, where their combined set of expert knowledge matches the problem best. The one person must be a jack of all trades, and necessary a master of only a few, due to humans' limited lifespan.
(I'd say it's a bit like a multi-threaded program: very often one thread is not enough, but only a few threads can do varied but coordinated tasks. Massive multiprocessing only works when coordination between peer threads is not required, and usually they do the same embarrassingly parallelizable thing anyway.)
Additionally, a team of one has no redundancy. You know, how Bob is responsible for maintaining the business critical databases, and now the databases are on fire and Bob is on a canoe tour through Canada without a phone. Oops.
For business critical things, you generally want 3 guys who can replace each other competently. Three, because one is none if the one guy gets sick, and two is also one if the other is on vacation and thus none.
You jest, but we've had weeks during which - out of 6 people - one was on vacation, two were sick, and then two more had to call in child-sick because their respective day cares had to close due to positive covid tests. And suddenly you're last man standing between the outage gremlins and customer systems.
While one person per team is ideal, two are better because they can discuss ideas and improve through arguments. If their views become too extreme, having a third person is better to moderate the views. Fourth person is needed so that one person wouldn't be a pariah holding minority view, and you want a fifth guy to make sure in 50/50 split, a decision gets made.
Five people need a lead that keeps track of everything, maintains the goal and resolves conflicts. Of course, two leads are always better than one because they can discuss ideas..
One person teams are only best when it comes to coordination overhead. they have obvious deficiencies in terms of "bus factor". If the one person in the team leaves the company, gets ill, dies or takes a vacation, work will have to be transferred to someone else (in which case the coordination overhead returns) or the work has to stop (which is not always feasible).
A lot of the inefficiencies of bigger companies come from the redundant people needed to fill out the bus factor. Having a DBA on hand for emergencies might be overkill for a startup, but when you have revenue of 100 million per day depending on the database always being up it is much cheaper to have a spare person sitting around (even if they're not doing much most of the time).
One person "teams" but reviewing and approving each others' work and then switching every six months or so, works well.
From what I have seen single person can deliver more and quicker than 2-5 people and having to seek approval from their peers keeps quality in check too.
In that case you don't have a one person team. You have a distributed team with effective communication and well designed systems that a single individual execute with minimal coordination. Which is the ideal but calling them one person teams is a misnomer.
I think you want at least two to cover people missing obvious solutions as well as provide a second pair of eyes when finding issues.
And because two means if there is a reason one person isn't around loses the benefits two people gives you then I'd argue three is the minimum size you should aim for.
