Remember, that when you're spitting out constructors, the rest of us can build warships. Maxing out a SB with all modules in the mid-game can cost you 1500+ or more manufacturing points, which is more than enough to build 3 or more pretty buff medium warships.
Even triple constructors cost 150+ (if you want them to have decent engines), so it takes at least 3 to build a SB up to mid-game max. That's the cost of a tweaked medium warship.
The absolute maximum number of SB per planet is 5, presuming you optimally place them. If you've got a couple of planets nearby, that drops radically if you want to have multiple planets per SB, and if not, it still drops per planet due to placement overlap issues.
Honestly, a good rough estimate I see is a max of 3 SB per planet, covering all possible planet location/combinations.
Consequently, I'm only building multiple SB per planet where it's really worth it. About 50% of my planets get a single SB, and 25% get 3, and the rest none.
SB spam is really only a "problem" in the game when you've managed to get left alone for a very long time, and avoid building much of a military. Because building constructors means I'm not building warships, and that's a suboptimal tradeoff unless you're only at peace for long stretches of time.
Note: my reference point is playing on Gigantic or Immense maps, moderate number of planets (typically, less than 100 on the whole map; everything set to Occasional), with 6-7 AIs and slightly less Minor races.
