Karazhan Guild Run

80% of the maximum size of the raid must be in your guild. So in this case, 8. However, I did hear somewhere that Blizzard lowered the required amount for all old raids, regardless of size, to just 8. I can't seem to find confirmation for this, though.

The percentage that was reported is really not accurate. Because as you have found out, 4 of 4 raiders is 100% and should have been sufficient. However, the REAL limit is 80% of the INTENDED number of bodies for the instance. As this is a 10m raid, you need 8 guildies 25m raids will require 18 guildies 5m heroics will require 4 guildies The quick guide is on your mini-map. It tells you the nature of the instance you are in (5, 10, or 25) and if it also has your guild flag, it is a guild run (if you mouseover, it will leave nothing to chance, it says whether or not it qualifies in the tool

Eight is correct for the minimum number required. Confirmed this in several of the old raids.

Also, the minimum required encounters to get this achievement are Moroes, Opera, Curator, Chess and finally Prince "You face not Malchezaar alone, but the legions I command."