How many angels can dance on the head of a pin?
Now don't laugh This was a "serious" question at one time and reputations (and livelihood) were made and destroyed by how you argued the issue.
Here, the Captain has posed the exact same question in the form of how many trips can you make before, yadda, yadda.
The real joke in his question is that whatever you do is allowed until the instant the FAA/DOT says it is not allowed - period.
You can be a world famous and uber competent aerobatic pilot until the instant the FAA says you are not - Bob Hoover - because you 'dissed' a couple of inspectors by not licking their boots.
You can be a permitted Part 91 flight until the instant the FAA ramp inspector says that the rivets in the frame of your aftermarket sun visor do not have the required paper trail back at the manufacturer to be certificated rivets.
Now, you can puff up and and righteously intone 'ow in ell am I spozed to have known that?"
And the inspector will quietly put his finger on the paragraph that says the PIC is responsible for determining that the aircraft is legal to fly - and shrug.
That's a big ten-four gotcha, buddy.
And if they are looking to get you no amount of arguing that you could not have known about this or that will make the slightest difference.
The reg's clearly say you HAVE to know.
Now, the real answer to the Captain's original question is that the pilot's best defense is ignorance (already cited by several).
You loaded the passengers. None were obviously intoxicated, carrying visible weapons, no naked screaming sex slaves were dragged aboard, yadda, yadda.
So you did your W&B and flew the plane (shrug).
Let me pose a more likely question. You are the commercial pilot hired to fly the C-180 dropping skydivers at the local jump port. Now, the big guy with the cute girlfriend is paying her jump fee back at the office and she is paying him back with sex...
Are you legal to fly them?
careful now