This is something that happens almost completely off-page, but it’s really really unclear what exactly happens and I’ll be following up on this in Tyrion X of ACOK.
There’s a big bit of confusion in that Renly claims to have 80,000 men at Bitterbridge when Catelyn meets him there (60,000 foot and 20,000 cavalry), but in the next Catelyn chapter has him claiming to have 100,000 foot in addition to his 20,000 cavalry.
Then Randyll Tarly rides for Bitterbridge. The next time we see the Tyrell army after Blackwater, they have somewhere between 50-70,000 men, with most estimates coming out to 60,000.
Now, given these wildly varying possibilities, it could be that almost no-one died at Bitterbridge – if Renly had 60,000 foot and Tarly grabbed the lot and just killed some officers. If Renly had 100,000 foot, then Tarly must have killed 40,000 men, which would be the largest single death total in any battle in ASOIAF world history.
Even for a massacre, 40,000 is improbable, so let’s see if we can firm some things up. Given that we know Mace brings up his 10,000 from Highgarden, that would suggest that only 50,000 of the Tarly army at Blackwater came from Bitterbridge. To me, this suggests that Renly was full of shit, and had 80k rather than 100k men, 60k men were left at Bitterbridge, and Tarly had 10,000 men – mostly Florents, but also Fossoways, Cranes, etc. – killed. Which would put the massacre up there with Oxcross, but not way ahead of it.
However, this leaves some open questions:
- given that the Reach has 100,000 men on its own, and the Stormlands at least 25,000, if Renly only had 80-90,000 men tops, where are the remaining 35,000?
- How many of them were Stormlanders? Given that the Stormlords divided on whether to support Renly, I think only 12,500 men fought for Renly from the Stormlands, which would suggest that there’s another 12,500 who might fight for or against Aegon.
- If that estimate is right, then 77,500 men of the Reach fought with Renly, which would mean there’s another 22,5000 men in the Reach going toe-to-toe with Euron in addition to Garlan’s 30,000. (which makes Euron’s crusade absolutely insane – even with one hand tied behind it’s back, the Reach can field 50,000 men against him)
- What’s going on with the cavalry? The Reach should have 27,000 cavalry by itself, and the Stormlands another 6,750. Now, even with only half of the latter backing Renly, that’s 30,0375 cavalry – but Renly only had 20,000? Why did 10,000 “knights of summer” agree to hang back with Mace?