Any ideas on what that may be? I've seen systems where there have been more FCs than planets in the system, I'm sure you have seen the same thing.
The limit is the total number of objects that can be in a system, minus the planets, stars stations etc and any ID64's reserved by FDEV for future use. That number is either 256 or 512 depending on whether my recall is correct. It's based on the maximum number of 64bit ID's available to a system, which is a 9 bit digit, I recall hearing one digit was reserved which would make 8 bits available and 256 objects, but if my memory is incorrect and it isn't reserved then it's 512 objects. Because FC's are visible in all modes and platforms they have to have their own ID64, which is why moving them takes so long, because that ID64 has to be updated across all platforms. It's also why you can't stack them in the system map at the moment, although there may be a way around that.
The limit is IIRC 16 per body in the system, so a system with 1 star and one planet could have 32 carriers, obviously things will get cluttered in a 94 body system.
There are systems where you can't put anymore FC's even though there are free orbits, the limit for stellar objects has been reached, and no it's not 16 FC's per body I have seen more than that, I believe it could be in the range of 26 but I haven't seen any solid evidence for a limit per body yet but I am assuming they use the same orbital slots as moons which are naturally limited by the letters in the alphabet, but there will be some sort of database field limit. If it was 16 the field would be 5 digits, which makes sense in a binary systems, however I have counted more than 16 around a body so we can probably assume at least 6 digit field and 32 slots available for FC's.