Communication is very important on a project. Project manager is communicating 90% of the time. Project Manager is responsible for good communication among team members. In communication there are two entities, sender and the receiver.  Receiver is most important. Communication channels is very important subject in PMP studies. Communication channel is a path between the team members.  These members belongs to same team.
Formula for determining the number of communication channels on a team.

Communication Channels = [N * (N-1)] / 2 where N is number of team members.

In case there are 3 team members, then number of communication channels are [3* (3-1)] / 2 = 3.

You add one new member. Total members of the team are now 4, then number of communication channels are [4* (4-1)] / 2 = 6.

So by adding one additional member, three communication channels are added. It is a very important to note that by just adding one more team member, your communication channels have doubled.

In case there are 6 team members. Then number of communication channels are [6* (6-1)] / 2 = 15.