Lot depend on why you are keywording.
Just for yorself to find images , then a small nummber should be ok, if your doing it to post on the web and to help others/companies search, then the more the better, so long as they are relevant to the image.
For yourself, and you want to find all your football shots and not your rugby shots, then one keyword 'Football' would do the job.
To help others search, then Football, sport, goal keeper, Manchester city, liverpool, anfield, and so on.
Keywording is the one thing that is poor in Aperture, as I understand it in Lightroom you can do hierarchical keywording so that if you enter say Old Trafford, it would automaticly add Man Utd, football, Manchester, England and so on.
If your looking for list of keywords and are prepared to pay then
Controlled Vocabulary sell 11,000 terms organized in a hierarchical structure.