First if this is posted in the wrong area, I'm sorry.
My question is how many points should be allowed if I ok 18 for one ability. Im creating a post Iron gods campaign. I want the players to have their new class with the 18 in the most important attribute.
Depends on your initial point buy, but just increasing the total points may not achieve what you desire because not all classes are single ability dependent.
a flat +5 points effectively reduces that 18 to cost 12 points, but if your initial PB is 15 I'd probably still not buy an 18 unless I was playing a fighter or wizard.
Flat out giving the players one 18 and a reduced point buy (say -10)would work for most classes, but would limit options for classes that like a more balanced array.
Thats all a long way of saying start at Epic - 25PB pretty much guarantees an 18 for any class that wants it, but gives a lot of versatility for other options.