Costa Rica has 2,654,627 million registered voters, 1,061,851 million of which need to turn out (40%) in order for the referendum to be binding. The majority of that wins - a single vote difference in the 4,932 polling stations could make the difference. So, for the "no" vote to win, a minimum of 530,926 + 1 people need to vote that way (assuming the total of 1.04 m turnout). Since there were 150,000 people that marched in the "no" march last weekend, this means that each marcher would need to turn out a minimum of 4 of their friends and family members to vote "no".
If less than 40% of the voters turn out, CAFTA will be kicked back to the legislature, where the pro-CAFTA president's supporters have a narrow advantage. But polls are showing that 60% of registered voters intend to turn out, so that's unlikely.