A DigitalPoint Forums thread asks what is the keyword limit per campaign within the Yahoo Search Marketing ad marketplace?
Based on the YSM help section, you can have up to 1,000 keywords per ad group and up to 20 ads. But we learn an additional item from the thread. It appears that when you initially add the ad group, you can only add 500 keywords at that time. Then after you save your ad group, you can add an additional 500 keywords to reach the 1,000 keyword limit.
Forum discussion at DigitalPoint Forums.