The quote I was referring to is this one from the latest update thread (#449):
(Emphasis are mine)Originally Posted by Sparton_LOTB
I think this is the only one where anybody from the devs ever spoke about absolute quantaties. I do remember the quote you are talking about, though. I think Sparton mentioned rewards being doubled if the minimum requirement for a tier was met twice (e.g. 1000 instead of 500 for tier 2 or 40000 instead of 20000 for tier 6. However, I have no idea how to find it with this forum search.
The fifty rare souls are a possible reward for a tier 6 sacrifice. I got them for my first sacrifice ever and even named them in the above mentioned thread (the one that didn't work as an overview ). I consider them a really good reward taking into account that the chances you get garbage from all of them is pretty low - especially compared to 3-4 G/A Souls giving you only 3* or a legendary giving you another Magus Golden Son (the latter one is ca. 60% for me).