But it sure seemed back in Warzone Fenris vol 1 that there were more than 100 returning Wulfen. That'd only be what, 8/9-ish per Great Company, less than a full pack (less if you count the first set that Harald picks up, yet all 12 Companies still have Wulfen to find). While ragging on GW for being bad a scale is nothing new, that seems implausibly few for the shenanigans they get up to (ignoring for the moment that the entire 'released by Magnus for scheme' thing is complete too), especially as multiple packs appear in the same place.
Let's says that 100 turned Wulfen and were released from the webway, 250 are in the webway, 200 lost on Prospero, 50 lost in the webway and another 100 who didn't enter the webway and stayed with the legion. That easilly makes up 600 with some of the numbers being off.
I don't know, it just seems that 'Ashes doesn't tell us what happened to the entire 13th Company' seems like a better explanation than trying to fit everything currently known about the post-retcon 13th into the (pretty small overall) contingent that went to Prospero.
*Blows dust off my 7th Ed codex* so this is the most recent fluff on Russ leaving.
"He rose from the great table, put down his drinking horn, and summoned his most favoured retainers. Of these, only Bjorn the Fell-Handed, youngest of Russ's Wolf Guard, was left behind."
So most recent lore states he only took his companions, minus Bjorn, and left. Nothing about the 13th company is mentioned on page 8 "The Dissapearance of Russ". So we have to take that Ashes of Prospero and Warzone: Fenris is correct, the 13th were in the pseudo webway, killed on Prospero, released as Wulfen across the Galaxy. The most recent lore has laid the foundation. I have yet to see any lore stating that it was the 13th that left with Russ, except fan stories.
As for what happened to all the 13th, I can not say the lore about them is in shambles. However the lore about Russ leaving is written and made clear.
"It has been said that the Lord of Ultramar sees little else besides the primacy of his own culture. For him, all worlds might as well be Ultramar." - Diplomat Pyke
"I would have expected that suggestion from Ferrus, or maybe even Horus, but never Russ. He has a hidden depth that one" - Primarch Fulgrim
"The Wolves were the Emperor’s red right hand–monsters in their own way–but how easy it was to only see the savage, to fear the axe and miss the subtle knife." - Prmomus