Thanks for the nice words, bungy52; 150 oil rigs?
What kind of map size are you running (1024x1024)?
I know I set the .grf so that at least 3 rigs are built (randomly - and kept). For number limiter I'd probably have to put in place some building rules which I haven't yet started because there is no good explanation how to do it in the .grf specs. I also wouldn't want to confine them to only borderline areas, because they come in handy for a scenario with more water...
If of course there is another - simple - way to control the upper limit of instances, it would perhaps be applicable.