To many, the answer to this question is obvious but there are some of us who take a bit longer to accept this fate. It is a question I was recently asked and below is my response. I understand how basic the information below is but that doesn't mean it isn't valuable. Please understand that having 9 productive, contributing planets is very important and, IMO, should be accomplished in the shortest time reasonable. I hope anyone out there that needs help with this subject seeks it out in this thread or through PM.
Well, running 9 planets at once isn't that hard. In fact, it makes the game a lot more fun because you can actually 'play' more... to a point. So let's take a step back and talk about how one succeeds in OGame.
There's no winning the game, right? So instead there are ranks. Your rank is determined by your number of points. For every 1k resources spent, you receive 1 point. For every 1k resources you lose (destroyed defense, fleet, or the like) you lose a point. So immediately the benefit of having 9 planets producing as many resources as possible become apparent. But OGame is designed to be an attack-first game. Basically, anyone who is a fleeter and knows what they're doing has an advantage because combat gives attackers an advantage over defenders. So shouldn't one just be focusing on building fleets then? Who cares about colonies and mines?
Well, this constant production is needed to build an initial fleet, replenish losses, build higher level mines, and research technologies. Plundering other players can supplement this as much as one desires. Some of the highest ranked players in the older universes make more resources through plundering and harvesting debris fields than they do through mine production.
No matter how it's viewed, the game increases the cost of buildings and research from 1.5 to 2 times the previous level. So after a while, these other means of obtaining resources are no longer a luxury but instead become essential to survival.