R0 is a statistical average, but even if we assume that every person infects exactly 5.7 other people, these two tourists would have infected 11.4 people. Those 11.4 people would infect a further 64.98 people. you're already pretty close to 100 after just two levels of spread.

The only reason it's limited to as low as 100 is that people are taking precautions to prevent the infection from spreading.