At the bare minimum you will need two people. One to be the Game Master, or GM. The other to be a player character, or PC.
For the best experience playing Atomic we recommend 6 total players. One person acting as the GM and 5 other friends who are player characters. This way when there are decisions to be made there is an odd number of players, so votes are simple. Also, this is just about the right amount of people to help and aid each other without the confusion of too many players trying to do to much.
The Atomic RPG System is specifically designed to force inter player dependence. No single player character should be able to tackle all obstacles. Only through teamwork will a Atomic group dominate all situations. Only through teamwork will a Atomic group achieve the optimal role play experience that all gamers hope to achieve.
It can be fun, once in awhile, to have large Atomic gaming sessions of 10 to twelve people. Though we caution against regular gaming groups of this size as it is often far too confusing and takes too long to do much of anything. If you have a group of this size that wants to play Atomic, we suggest splitting it into two separate groups.