How many copies of a card you need will also depend on:
1. How often you need it.
2. When you need it
3. How likely you are to draw it
So if we assume the following:
1. You only really need Rumor Mill to score the game-winning agenda (or when you do use it, you will trash enough defensive upgrades that they are unlikely to be an issue again)
3. You will have drawn most/all of your deck by then (against glacier this is fairly easy, and if Obelus + Hades Shard is in use, then you can draw your entire deck by mid-game if you want to)
If those assumptions are correct, then 1 copy is probably fine. This contrasts with employee strike, which runners probably want to see earlier in a game to turn off IG or CtM ID abilities early in the game to keep the board state in check. Now, I didn't actually look this up, so I may well be wrong.
Finally, there is the meta-consideration. Before Rumor Mill, many corps relied heavily on Caprice and Ash. Palana Glacier used to be a pretty strong build largely because of 3x Caprice. However, with Rumor Mill making those upgrades blank, they are hard to justify as many deck slots for, since when you really need them to work, they can be shutdown. Thus, many corps are running fewer defensive upgrades, and those that are using them, are dedicating fewer slots to them.
Those are my thoughts on the matter. Someone else may have a different take on it. But I think most players will agree that the very existence of Rumor Mill makes the glacier archetype much weaker than it was.