Arival/Destination how does it work?

I thought I saw a question about this somewhere and now I can not find it.
How does the arrivals work? Is there a set number of arrival slots or is it unlimited? I see that as you handle aircraft you get more arrivals from the rewards. When I purchase an arrival spot from the routes button ie. I am at IAD and purchased all the medium arrivals from the other airports, does that add more arrivals to IAD or do I still have the same number of possible arrival slots. I have the terminal with all the medium slots open and all the slots purchased but only like 5 of them are in use at any time.

More planes come to your airport.