No, I need 10 of them overlapping. 5 is when you're trying to attack one of my outer planets. If you try to hit my home world, you need to fly through at least 20 PDs before you even get in range of its own PD.
And that's exactly how I want it. Because I spent months building up that defense, it should be equally hard to get past it.
Well, nothing would prevent you from still keeping all of them. You just wouldn't be able to field much fleets, but that'd be the price for going the safe turtling way. Right now you've got no downside to essentially building yourself an invincible fortress. No matter how you look at it that's not balanced in any way.
Not sure how many players have actually TRIED taking on a full PD defense, it is a lot more intimidating in ones mind then it is in reality.
I took one from Hierulf that had a single PD. Didn't even have Long Range sensors 2 on it so his visibility was quite limited and I sailed through half of the PD defense range with no problems. Still, it was expensive enough that I can't try it again and he didn't even try to actively defend it. The fact is I'm almost surrounded by a Great Wall of PDs from him and I absolutely can not touch him in most places. It's not because of good tactic or play on his part. He just spammed the build button on one upgrade. It's not even a hard choice to make.
The fact you had to send out 15k battleships and lost 13k of them against 9 PDs and are now still left with 8 more to fight should tell how horribly imbalanced they are.