The Scaled Agile Framework (SAFe) implements the Dunbar’s number in Agile programs to decide the upper limit of the ARTs and an effective Agile Release Train consists of 50 - 125 people (5-12 teams). This suggests that ARTs with these many numbers of people can have a stable, effective social relationship. There are industries having larger trains, but it has the disadvantage of restrictive rules, bureaucracy, politics, and unnecessary management layers.
(Before reading this, please have a quick look on the first part of the blog: Dunbar's number: From Prehistory to 21st Century Enterprise)
Agile Release Train is a long-lived, self-organizing team of Agile teams set to deliver some set of values by planning, committing and executing together through program increment (e. g. Maintaining a Cadence). The default program Increment is for 10 weeks, shorter is 8 weeks and longer is for 12 weeks.
Let’s divide the ARTs by the number people that are manageable in those:
50 to 80 - In ARTs with (50-80) people have a well-balanced system, because of minimal leadership, restrictions, and conflicts. All the members of a medium sized (50 - 80) can have clear transparency over what is happening around. What's more, the collaboration among the members is also very high. Many organizations and top notch Agile leaders consider this size as the perfect size, where leaders can lead with alignment, cross-functionality, and self-empowerment. Such as the founder of Virgin, Richard Branson thinks 60 people is the right size for an ART.
150 - There is a factual industry support behind this size of ARTs. Many companies in past have found that ARTs with a few hundred people are the most effective. This is also the biggest size of Dunbar’s layers. That means this is the optimum limit and more than this would be too much to handle the business on a personal level.
There are industries with ARTs over 500 - 1500 individuals. But you may face many big challenges with this kind of teams:
What the Prominent researchers say about Dunbar’s number: