There's a 1000 keyword per ad group limit. I can't post URLs yet but it's in their FAQs here: http://help.yahoo.com/l/us/yahoo/ysm/sps/faqs/camaccstruc/ads_number.html I'm not sure why you're having problems at 500 keywords. 47
You add 500 keywords max at initial ad group setup and after the group is set up, you can add an additional 500. You are allowed 1000 keywords per ad group. I set mine up like this: Adgroup1: 1000 keywords Adgroup2: 1000 Keywords And so on. Hope that helps.
Too many to list. Some people use over 30,000 keywords for one offer. I tend to stick to 200-500 KWs though. I will expand to a lot more soon though.