None. I recommend 2 planets per galaxy. One in the lower systems (1-249), and one in the higher systems (250-499). This leaves you 3 planets for floaters.
Greatness isn't bestowed; it's chased. And only the best catch it.
Nooo... you find yourself with a floater sitting next door and 100 rips hitting you every half hour(or 2000 bs and bunch of transports with a 8 min attack time, depending on whos doing it) one planet after another. Just like Rodikus did. If your def is low enough they might send little fleets to all your pleants at once ad hit you 4 times each. bye bye any fleet that is down and most of your rez.
Again... unless you plan on being a complete turtle (and that is no fun) SPREAD EM OUT!