I agree with pistolwink and it also probably depends on the program. I mentioned at least 2 names in each statement and got in in 4 of the 8 schools I applied. before doing that I contacted all of these potential advisors to make sure they will be taking new grad students this year.
and mentioning many names also doesn't have to be necessarily bad. in one of the schools there were 4 professors that work in my area (which is not usually the case, only 1 or 2 in the other departments) and so I used this to say something like 'the fact that there are so many people doing research in this area, such as prof x,y,z, and w makes your school my top choice'. I was accepted, contacted by one of those whose name I mentioned and she was very excited about my application.
so there are no rules of thumb, I guess