As many as you can handle whilst keeping post quality up while avoiding spamming. Keep in mind, the only real way to avoid spamming is to think critically about posts and topics you respond to.
When did post quality become a thing? Have you seen the posts by NotLambChop and the numerous new accounts that seem to follow him around/occasionally post on his behalf? I actually find myself not checking threads if the last post is by NotLambChop. I honestly think his paid day job is to deter people from visiting the forums.
Yesterday, I ignored 3-4 new accounts, all seemingly the same person, likely NotLambChop, trolling in the Speculation sub-forum. It's getting boring fast.