Likewise this doesn't have any stats for individual boards, but it's something:
https://www.4chan.org/advertise
Unique visitors per month: 25,000,000
Posts per day: 1,000,000
It might be a little outdated though. The page says the Alexa Traffic Rank is 836 (Global) and 371 (US), but if you check Alexa yourself it says 4chan's rank is 729 (Global) and 353 (US). No idea how much it swings around in any given timespan, but a difference of over 100 in the global rank seems to suggest the info on the 4chan advertising page is old.
Some unofficial board archives have further information like post count for the last hour or daily activity per board.
You could just try emailing moot.