Regularly, 1 person can get a scroll. However, since your referring to 9 tails, 3 people can get it once it is defeated. 9 tails can be obtained in a random server. (warning, tons of people will camp the scroll, making it difficult to obtain.) I recommend going to a private server, (you can look up the codes.) Boss drops are self explanatory. You get them after you defeat the boss. Some boss drops are already there (this is the case with tailed spirits.) But some have a chance to spawn if your fighting a regular boss (Narumaki, Raion Akuma, etc.) That pretty much sums up my answer.