at work i deal with "cases", Some of these cases are transferred in to us from other organisations. The case is then known as a "transfer in". If i have several of them, would i call them transfers in or transfer ins?
I'd call them transfers in, but organisations, or even individual offices, often have their own jargon for this sort of thing. (They might just call them TIs or whatever.)
I think you could argue that either is correct. The first, "transfers in", is in the spirit of treating "in" as an adjective, rather like the "major" in Sergeant Major, whose proper plural is sergeants major. On the other hand, "transfer in" reads like an entire noun, so that you'd add the plural s only at the very end of the noun, ie "transfer-ins".