Don't forget to add some safety margin for the missiles that will reach the hollow parts of the base and not deal any damage
its not really a question of how many missiles you should bring, its how many missiles should be AB. If you are taking a stealth bomber, its a hunter/ AB ratio. I usually determine that off of several factors
1) are there any other small targets along the way I can bomb by myself 2) how many other StealthBombers are hitting the same target 3) how long do I reasonably expect to stay alive
usually i find 2-4 adequate for participation in a coordinated run. If I have to walk a longer ways, I will bring 6-8 to get any targets of opportunity.
also, BRING HUNTERS!!!! Whatever space you DON'T use can cart hunters. That way the pesky scout trying to catch you will have something else to worry about.