1. A Contested Election
9 Justices are needed for the Court to function properly, if the decision of the justices are tied there must be a tie breaker! What happens if the election is contested and it must go to a Supreme Court with a vacancy on it which goes into a tie?
2. More Justices, More Problems
The more justices we have, the harder it will be to get anything done. Too many members means too much time, too many disagreements, too many people to have presiding over a case. Have you ever heard of the US Congress?
3. Everyone Has An Opinion
Adding multiple additional justices only means each one's opinion on the topic at hand is that much less crucial. It would be much easier to get multiple vote opinions on the case, rather than being close and every justices opinion matters.
4. Diminish Constitutional Importance
More justices would mean each President's Supreme Court Justice pick(s), are not as crucial to the country like they are with a limited low number of justices.
5. Timely Decisions
Any disputes on election day, like such during the Gore v. Bush election of 2000, would take an extreme amount of time to come to a conclusion with numerous more justices. With a total of 9 justices, they can discuss their points and opinion, and come to a conclusion much quicker to solve the issue at hand!