The not knowing how many gold saints are out there is a point the anime makes to justify that entire thing, but Milo in the manga isn't surprised? He's surprised that they found the Sagittarius cloth in Japan, in that page he is surprised to see almost all the cloths there, and I think that's just a way to present them to us.
They're not even surprised when he uses their Gold saint titles, and Aiolia is mostly wonder what is he doing there.
For Mu I could justify it with the fact that he went away from Sanctuary when he was 7 and never came back and majority of Silver saints are younger than him, so they probably knew him only as the cloth repairer that doesn't let anyone in. Probably the fact he was a gold was kept a secret to keep people from having doubts about why a gold saint wouldn't go back to Sanctuary.
Aiolia, Shaina knows? At least in the manga, it's been a while since I watched the anime, so I don't remember if they did a mess on that front as well, and Seiya, in the manga, didn't look surprised Aiolia was a Gold saint.
Shiryu, as I said for Seiya, in the manga isn't surprised either that Dohko is a gold saint, he doesn't acknowledge it either tho. And while we want to try filling a plothole done by the anime, I think it would still not make sense, because I'm pretty sure he was hoping he would be part of the people who went against Sanctuary while training him, since he also reprimanded him a little for saying that he probably wouldn't have wore the cloth anymore.
Also Saori at the start of the manga explains pretty well that there's twelve cloths. Hyoga we can justify it with the Crystal saint true, in the manga I think it's implied he knows about Camus's status though. There's a flashback of Isaak and Hyoga talking about the fact that Camus seemed worried and doubtful about Sanctuary and its situation, but I don't remember how well they knew his gold saint status, but when he went against Camus in the Libra temple he didn't look surprised