Truckloads of info on this subject on this forum, do some searching around and you'll turn up alot of good info. Opinions vary greatly but the 4 pellets in a 6" circle as stated above is about the minimum you want, the "magic pellet" as the say sometimes hits them in the right spot but not always and I don't count on it.
One huge factor in this debate is the type of pellet your using, weather it be lead, copper plated lead, tungston, etc. The more expensive tungsten based loads carry more energy at further ranges, therefore have better potential. You may have X amount of hits at 50 yards with lead but are those penetrating? Something to think about.
I pattern mine at 35 & 50 yards with tungston based loads on posterboard. I like to see 100% pattern on the 35 yard targets and 80% or greater on the 50 yard target.
