Before Damnation of Pythos, I would've guessed 50... now, it could be anything.
30 novels in and we still haven't gotten a novel focused on the DG, IF, or NL. Novellas... but not full novels to truly flesh out these legions and how they're different.
CoTE's list of events seem pretty good...that's still a LOT of stuff to cover... and that's all before the siege.
I would add the Lion saving Russ from the AL. That needs a novella at least.
Agreed that most Legions need repositioning. Some seem like they're (almost) ready to go. After Scars, I don't think another novel is really needed for the WS. Their arrival on Terra can be a small part of another novel. Of course, an entire novel can be written about the WS's actions on Terra.