Best answer
Meta does not publish a fixed number of automated DMs per day, so anyone quoting one is guessing. What actually limits you are three mechanisms, and all three are visible while they happen.
The 24 hour window. You can only send free-form messages inside 24 hours of the person's last inbound message. This bites long before any volume ceiling.
Rate limits. Send too fast and the Graph API answers with a rate limit error and a retry-after hint. ChatinFlow reads that hint and puts the channel on a cooldown for exactly that long rather than retrying into the same wall, so a burst slows down instead of failing.
Spam and policy blocks. If Meta decides a pattern looks like unsolicited messaging it can block the channel outright. That arrives as a specific error and the channel stops sending until the block clears.
So the practical ceiling is behavioural, not numeric: messages people asked for, inside threads they opened, at a human pace. A comment to DM flow that answers two thousand commenters over a day is normal traffic. The same volume sent cold is what triggers a block.
If a channel does get blocked, it is not permanent and you do not need to rebuild anything. ChatinFlow retries the channel after the block ages out and brings the paused automations back with it.