"How many knives?"
Translation:Cia mheud sgian?
You do use the singular when asking "how many" ("cia mheud sgian", "cia mheud bus", etc).
If you're talking about one or two of something then you also do the singular ("aon sgian", "dà sgian"). When you go to three ("trì busaichean") or if you're saying something like "the buses are green" ("tha na busaichean uaine") then you go to plural.
It works the same way with "co mheud" (personal preference to use "cia" or "co", from what I can tell; Ciaran and Joanne have talked about it before but I can't remember what they said, just that both are common).