The short answer is - the number of two-way radio channels you pick should be the same as the number of teams in your operation who will share communication.
Imagine you are responsible for organizing communication at a large resort complex. For safety you will need to have one common channel for all employees. For normal communication, however, you'll want to assign one channel to security staff, another channel to house-keeping staff, another to recreation supervisors and so on...
Channels divide two-way radio communication, so that users can communicate with their team members and not be interrupted by conversations in a separate division.
Your two-way radios will come set-up with a particular frequency on each channel. Remember to check the frequencies in your area and re-set your channels to frequencies that are not being used by others nearby.
A word about Privacy Codes - Quiet Talk Codes (QT), as they are also known, are not actually private. The use of "privacy codes" simply filters out communications from others using the same frequency. Anyone on the same frequency could still hear you so long as their radio is set to not filter for a "privacy code".