I believe at the moment there is a bug on how many relations returned, which would be whatever your default response setting is set to (default without modification is 100) but we do consider this a bug. Technically there is “no limit” (I say that with a grain of salt, as it depends on the maximum ID length of the databases table/collection.)
Performance however is a whole different story, I would suggest that with massive amounts of relations you either use the oneWay or manyWay relationship types as you shouldn’t need to get the data on both sides with that many.