...the number condition appears to work the same way...
NumberOf and Number are two different conditions.
Yeah, probably, but it seemed like it wouldn't be very useful if its so gradual
The rate can be varied a bit, if necessary. A single building that reveals the locations of all or many of the systems in the universe is a rather drastic change, and something between that and nothing is probably good to have.
On the other hand, the subtle, yet annoying lack of any stars appearing in a nearby area would probably encourage the player to go explore there with his scouts.
Depends on the galaxy shape... it might not be obvious that there's a hole in the revealed stars.
I think the real reason the counteracting building isn't necessary is that its not really a bad thing if your opponent has basic visibility of your systems from far away. It doesn't reveal your location at all.
Revealing system locations does, as you note, encourage exploration of those systems, even if a player doesn't know they're populated.
Also, there could be effects that make systems less stealthy only if they contain objects of certain types or with certain owners. Counteracting effects of this sort would be important.
If there are buildings which increase system stealth (and I believe there should be), they should also prevent visibility of starlanes leading to them and propagation of visibility along adjacent starlanes, so that a scout exploring a nearby part of the galaxy wouldn't be able to detect the system unless it actually entered an adjacent system. Probably only useful for empires holed up in a corner of the galaxy going for tech victory, or some really crazy kind of espionage victory where nobody even knows you exist.
Starlane stealth is something I've thought about, and would like to have, though don't plan to add soon. One problem with it would be how it would interact with adding or removing starlanes, which I'd definitely like to add. If a player sees a starlane one turn, and then another turn doesn't see it, but still has the same visibility level of a system connected to that lane, it could be because the lane was made invisible, or because the lane was destroyed, and there's no obvious way to tell the difference. In that case, what should the UI / player assume?
Hopefully it can be made to show only stars, one at a time, not repeating any stars, and no starlanes.
I don't see repeating of stars as a huge problem. The rate of useful information being revealed would fall with time, as a larger proportion of the possible targets are already revealed... This doesn't mean it's useless.