Why not 7, like on the Florida Supreme Court?
Because Congress thought that 9 was a good number. 
Originally, there were 6 justices on the Supreme Court. That was because there were three judicial circuits in the country and two justices were required to travel each of the circuits hearing all sorts of Federal cases, most of which were fairly petty.
The number of justices was reduced to five right before Thomas Jefferson took over because the outgoing Federalists didn’t want him to appoint any more justices than he could right away.
Slowly, the Court worked its way back up to 9 members as the country expanded. In 1863, Congress added a 10th circuit and for three years, there were 10 justices. Then once Andrew Johnson took over and succeeded in pissing off Congress, it reduced the Court to 7 justices. Once Johnson left in 1869, the number went back up to 9 where it has stayed.
Presumably, nine is a workable number and changing it now is probably not something anyone would want to see. Ask Franklin Roosevelt.
Good summary, {b]Bob**!
One of the concerns about the present federal court system is the substantial narrowing of opportunity to be heard by the nation’s top court. As the population grows, more cases are heard. And although the various District courts and Circuit Courts of Appeal have grown in size (numbers of judges, staff, etc), the Supreme Court continues with the same number, the same schedule, etc. This means that roughly the same number of appeals to that court can be heard, out of a VASTLY greater number of potential cases. Indeed, under Mr. Chief Justice Rhenquist, the Court has been scaling BACK on the number of cases heard in the last few years.
One proposal being floated is to roughly double the size of the court, then split it into two panels, allowing double the number of cases to be heard. Presumably, if so done, the Court would also then be allowed to rehear decisions of one or the other panel by assembling all 14 justices. There are, of course, logistical difficulties with such a system, and traditionalists make it hard to implement any changes. For goodness sake, you still can’t videotape it for posterity, let alone the news.