I can think of no legitimately ethical investment that ceases to be profitable based on the number of investors.
I can, assuming you mean amount of invested capital rather than just the number of investors:
A bank. A bank takes in at-call deposits and then loans funds out long term, for stuff like people buying houses. All their liabilities to depositors are covered by assets (loans), but if 50 % of customers suddenly withdrew their money, the bank wouldn't be able to provide it because they can't suddenly make their borrowers sell the houses they bought with the bank's money and pay it back. They would suffer a liquidity crisis, even if the bank is completely sound.
It's the same with pirate. Everyone withdrawing all at once and causing delays in pirate being able to pay out does not automatically mean he's running a Ponzi (although a Ponzi would show the same effect). It could could just be that his assets are not liquid enough.