Yes the limit is per port, and some thing that didn't occur to me earlier is that, the cable itself has multiple strands, so it is possible for an operator to attach each strand to a different port on the OLT. There is no significant additional cost for that. Then he could connect different customers to different strands depending on the load.
My point being, the operator wouldn't need to run a separate cable to serve a dense area.
Probably it's obvious, but it didn't occur to me until recently and was always thinking that if the link is saturated, the operator would need to run a totally separate cable and hence maintain multiple cables.
The cost of running or laying fiber is higher than the cost of cable itself. So operators typically use cables with twice the number of fiber stands required.