In Jet Age 4 the new slot system was introduced and carried over to other worlds. Instead of a static number of slots with slow growth over time, the game starts out with 50% slots and grows to 200% of slots over time. The growth happens rather quickly and then tapers off until reaching 200%. I'm not sure if this is still the case, but this is a ballpark estimate of what to expect. ATL I believe has ~80 slots at 100% and right now they're at 66, so I assume we're only at ~80% slot growth so far and have a ways to go.