Scrum Team Size

However, is the phrase, "10 or fewer" not rather ambiguous?

The Scrum Guide is saying "typically 10 or fewer," not "typically 11 or fewer", or "typically 9 or fewer". That seems fairly unambiguous to me.

You seem to miss the point that Product Owner, Scrum Master and Developer are not job descriptions.  They are roles with specific accountabilities and responsibilities.  It is possible that one person can be fulfilling all or part of the Product Owner and Developer accountabilities. Or any combination of the 3 roles.  So, yes it is possible to have a Scrum Team of 1 or 2 people if the Product can be supported by that many people. 

From my experience, those situations are not typical. There are usually more people needed to fulfill the accountabilities and responsibilities for those roles. I also fairly confident that "there are studies that show" the effectiveness of different team sizes for various activities. 

As for the Scrum Guide revisions, the last revision actually became less prescriptive.  Remember that Scrum is not intended to be solely for Software Development.  It is for any situation where empirical learnings can be applied to help complex problem spaces deliver solutions.  I know of Human Resource organizations using it.  I have seen it applied to Medical Labs.  I am aware of it being used in Event Planning and Sales organizations. The Guide needs to be minimally prescriptive in order to be applicable to a wide variety of possibilities. 

I don't think that the phrasing is ambiguous. Typically, or in most cases, a Scrum Team will be no larger than 10 people.

The Scrum Guide doesn't get into hard upper or lower limits, but it's possible to reason through them.

You can think about an upper limit at around the time it becomes difficult to complete the Daily Scrum within its 15-minute timebox. Although the Daily Scrum is for Developers, you can think about how many Developers can successfully review progress and plan their next day in a 15-minute timebox. In my experience, this is somewhere around 8-10. If your Product Owner and Scrum Master are not Developers, then your Scrum Team size would be around 10-12. This is very close to the "typical" maximum size of 10 people, so that checks out based on my experience.

A lot of the Scrum Events, such as the Sprint Planning, Daily Scrum, and Sprint Retrospective are about getting the team together to collaborate on something - a plan for the Sprint, a plan for the day, or improvement. When you have a small team, any kind of formality is too much. You want a team that's large enough to need the structures of Scrum. In my experience, if you have fewer than 2 or 3 people, the structures of Scrum add little value.

I would suggest that the optimum Scrum Team size is between 3 and 12. It's quite reasonable, realistic, and unambiguous to say that a typical or most common Scrum Team is 10 or fewer people. It encompasses most of the team sizes where my experience tells me that Scrum would be useful. Of course, in your context, you may be able to find value in Scrum in a 1- or 2-person team. You may also find value in larger teams of more than 10 people. However, you may want to start paying attention to potential problems that arise in teams larger than 10 people, and this is what the Scrum Guide is pointing out.

The Scrum Guide recommends having three to nine team members in a single team.

The Scrum Guide recommends having three to nine team members in a single team.

That is according to the 2017 Scrum Guide version.  The 2020 Scrum Guide version removed that recommendation and replaced it with the '10 or fewer' recommendation.



https://www.scrum.org/resources/scrum-guide