I believe it is something like 200 custom addresses per domain.
There isn’t a limit on the amount of emails it will handle
I believe this is a Gmail thing where people have reported some emails from the domain that Cloudflare uses to forward emails as spam, so Google is marking on the messages as spam. You can report the message as not spam to help.
If you really dig down deep into Googles help docs you find A LOT of limitations imposed by them. 200 aliases IS going to trigger Gmail automated algorithms FOR SURE. That means Gmail SILENTLY suspending or otherwise rate limiting.
If you need 200 or more then go with an email host who provides such quantity (Enterprise level in other words).