Depends on the test. Full-life trials, like this one, tend to have many more than that because you need to take into consideration a lot of the random factors of being alive. However, there other things to keep in mind here. The breed of rat used has a predisposition to get cancer in a real short time; more than half of Sprague-Dawley rats will get cancer within two years. As a result, they're used much more often in research centered around having cancer than getting cancer. Finally, the rats were allowed to eat food whenever they wanted, and however much they wanted. So, there's the possibility that one or more of the groups through chance just had particularly gluttonous rats. Because of this, you have a lot of questions as to what could have led to the problem -- was it the GMO corn/roundup, or did the rats feeding on GMO/Roundup corn eat more, or was it just (un)luck of the draw, etc?
Actually, in many countries it is really really hard to be allowed to do a study on a significant amount of animals due to animal rights groups, which have lobbied for committees who decide how many animals are allowed. Most times this number is way too low to do good science (numbers like 16 -- 8 main and 8 control mice, are not uncommon).
Some studies that purport to show certain effects do, and are routinely criticized for the same reason. Proper medical trials, particularly for, say, a new drug use far larger sample sizes.