Well, we have some new numbers on Second Founding chapters, at least of the Ultramarines, as per ADB"s great Void Stalker.
'It was the Ultramarines'. Talos lowered his head, surrendering to the memory now.
'A thousand warriors?' The Apothecary's eyes widened.'That's all?'
'You think in such small terms,' Talos chuckled. 'The Ultramarines. Their sons. Their brothers. Their cousins. The entire Legion, reborn after the heresy, wearing hundreds of icons proclaiming their new allegiances. They called themselves the Primogenitors. I believe their descendants still do.'
'you mean the ultramarines' kindred chapters?" Variel could almost picture it now. 'How many of them?'
All of them, Variel,' Talos said softly, seeing the sky once again on that distant day. 'All of them.'
With the more recent numbers, it makes sense to basically take the numbers of successor chapters for each legion and add a zero to the end of it.
Ultramarines would have about 230 chapters, after losing a lot of troops from their initial 250,000 at Calth and then restocking from local worlds as they expand during the Scouring
Most of the other Legions would have around 30 to 60 successors, due to taking losses but not rebuilding as quickly as the Ultramarines.
There are 21 listed second founding chapters for the other chapters, while Ultramarines have 16 named but 23 mentioned in total. Multiply that out, and you have something close to 440 successor chapters, 450 with the originals. The rey Knights number 666 thus would not be too far off in that scenario. The slight majority of Ultramarines successors over everyone else's also fits well with the 60% total of all chapters later.