Is there a formula that lets you know what max out if base number of aircraft is? I know max no increases periodically but what's that based on? It makes it very hard to plan.
Maximum is 1000, except in current MT where there is no limit. It usually increases by 10, later (think 2015 onwards approx) by 20. Might be only by 5 in very early days (think 1950s) iirc...
You can go to Office-> Base Airports to find how many Out of Base aircraft in use and the current limit.
As far as that growing. It's starts at 5 every January 1st until about 1990. In that time period there are a few random years with 10. After 1990 is is 10 per year from what I've seen.
After 2015 it starts increasing by more (20-25) from the old OoB cap of 600.