The capacity is in code, which gets compiled down into the exe and/or dlls. While you can theoretically have more than that in the save file, until or unless someone tries that manually in a hex editor we won't know how the game handles it. It could just stop checking at the limit. It could crash. It could even just magically work but not let you add more. Won't know until someone tries it.
That said, if we're lucky enough for it to work properly with excess nephs in the save, then it would likely be easier to create an import/export tool for nephs versus save files than to isolate and remove the code preventing over-breeding.