Automatically generated web search result categories were found to be beneficial in our previous study. In that study, we used 15 result categories and left the optimal number of categories issue deliberately untouched. To address this matter, we conducted a new experiment with 27 participants to compare search user interfaces with 10, 20, and 40 automatically generated categories. The results show that users prefer fewer categories. The use of fewer categories results in a slightly more accurate performance in result selection. In addition, the observed speed differences between the conditions were small. Although reading longer category lists requires more time, the users benefited from the increased descriptiveness, causing the overall speed to be almost equal. According to our results, the optimal number of categories is between 10 and 20.