5 team members results in 10 lines of communication, as shown in the following diagram:

If we increase the team size to 10 team members, the number lines of communication jumps to 45, as shown here:

The formula is as follows:

So, 15 team members would have 105 communication lines to maintain, 20 team members would have 190, and so on.

Different teams will experience different results, but at 10 or more team members face-to-face communication becomes increasingly more difficult. The Daily Scrum will begin to take longer and longer. Our team will either seek to keep it short and curtail conversations or run the risk of people disengaging as the coordination conversation begins to blow out.

Either way, important information will be missed and not everyone will be aware of what everyone else is doing and the opportunities for strong collaboration will start to become fewer. When faced with situations like this, our team will often start to resort to lower bandwidth forms of communications, such as documents, wikis, and email. Ultimately, this will slow things down.

To avoid communication breakdowns, Scrum advocates small cross-functional teams of five to nine. It happens to be around family unit size, which is where teams operate best when maintaining close connection and communication.

A small team means that each team member can hold a good mental model of what each other is up to, with a lessened cognitive load. Less load gives us the capacity to collaborate more closely. Being able to maintain high-bandwidth communication lines is what gives a Scrum team its speed advantage.

One final note, Robin Dunbar proposed that the maximum number of stable relationships that can be maintained by one person is 150. This figure is commonly referred to as Dunbar's number. This is worth bearing in mind as the number of teams we have working on a particular product start to scale.

Taking the median size of a Scrum team as seven, and then there would be an approximate maximum of 21 teams in the group before coordination between these teams gets strained, and we will need to find a way of further dividing the work.