p.13 Zone Tokens & Zone Cards
Place a number of Zone tokens equal to the number of players into the cup at the start of each round.
p.15 Scenario 1: Throw Down
At the start of each round, draw a number of random Zone cards equal to the number of players. Add one Acid Rain card to this pile and shuffle. Draw Zone cards from this pile very time a Zone token is drawn from the cup.
So in a two player scenario 1 game with three models each, you will have six character tokens and two Zone tokens at the start of each round in the cup. The Zone deck each round is made up with two Zone cards from the Zone card deck and one Acid Rain card (so technically three in total). Whenever a Zone token is drawn from the cup, you draw a Zone card from those three cards. At the end of the round (all tokens in the cup have been drawn), you start a new round by drawing two random Zone cards and adding an Acid Rain card (the Acid Rain cards from previous rounds remain in play if they have been drawn).
So each round there will be two Zone cards drawn and the game will most likely end in round, but not always.