Telling the truth, 15 construction ships for me is really much. I usually have 15 in a late game when I have more than 50 colonies. But is it a good to compare it? No, it is not. Feelotraveller already described how it works. For me it would be too much, somebody else would say that 25 is not eounght, and for You it may be ok. Just keep Your eye on maintandence to not go to hight and it should be ok
To be more precise about space stations (whether they are defence platforms, or mining stations) You can upgrade every station which is built over Your own colony. So it is not that You may upgrade Defence Stations, but not mining stations. If You will build Defence Station in deep space or over uncolonised planet/moon You will also be unable to upgrade it.
PS. Wouldn't it be better to change this theard's title from "Noob questions" to "Newbe questions"?
