The formula to calculate the communication channels is:
N x (N - 1) / 2
where N is the number of team members.
For example, if you have 3 members, then you have 3 x (3 - 1) / 2 = 3 communication channels. So if you have A, B, and C, then you have the following combinations:
A and B
A and C
B and C
Note that communication channels are only between 2 persons (no more, so you don't have to worry about A and B and C)
If you have 4 members, the number is 4 x (4 - 1) /2 = 6
For 5 it is 5 x (5 - 1) / 2 = 10
It is interesting to see how the number of channels increase exponentially with the team members.
You don't include stakeholders for the calculation of communication channels between team members, because they simply don't communicate with the team members.