That's kinda a desision you got to make your self.
If you have more planets you tend to advance in points very quickly. Also if you spread your planets apart then it makes res/fleet saving easier. However with more planets comes more stress cos you have to res/fleet save for another planet.
If I were you I would look and see if you have gotten into a routine with the planets you have, and see how often you get attacked/raided. If you get raided often then I wouldnt. I would worry more about spending time or res on protecting the planets you have. Whilst if you are in controll and don't get raided often then I would build another planet as it may liven the game up for you.
Now is were I contridict myself... If you are getting raided alot then your problem may be because all your planets are really close together. This would make res/fleet saving difficult, so you need to build a planet a couple of galaxies away. Remember if you are going really far yu will need a LC to help carry the deut needed to get there. Don't worry because the LC doesn't get destroyed along with the colony ship.
Hope that helps
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