I looked at this later, and my math should really converge. Should be 400 + 1 per million population, if we go that way, for a maximum of 1000 share holdings at 600 million, or a temporary manual cap close to that top amount.
For those unaware, the maximum an investment fund is even allowed to own of a given corp is 40%, and, again, can't exert any control over the corp. At 1000 share holdings, at most, the IF will, effectively, own the equivalent of 400 corps to reflect the collective wealth of 600 million population. This seems very reasonable, if the server would allow it, I would love to even see it higher.
Edit: Thinking about it, this leaves the investment fund cap effectively still less than the enterprise one, that can own up to 100% (though 100% would take it private again) of 500 corps. It makes so much sense.