It's not the number of people that are micro, it's the scope of the service... So what is the right level of manpower for a scoped service with unspecified operational demands? Unknowable.
The definition isn't small teams, per se, it's a small area of responsibility with singular focus. A lego block instead of duplo. That lends itself very well to small teams, but you could reasonably have 100 people working on a service and call it "micro".
Reason being: if those 100 people weren't working on their scoped 'microservice' they would be part of a much, much, much larger pool working on the shared 'product', 'platform', or 'service' that contains that exact same functionality, only without the clarity/scalability of application boundaries surrounding the individual service components.
That's not to say microservices are ideal, just that the size of "micro" is highly relative to ongoing operations.
I suppose it depends if those 10 people are purely developers or include the other teams you'd need - monitoring, devops, etc. For a critical, high-performance microservice, I can easily see it needing the involvement of 2+ devs (at least one senior), product owner, project manager, QA, devops, monitoring, etc.
If it takes 10 people to manage one service, it is not a microservice by definition. It is more like a 10x-microservice or a macroservice.