No 10% renewal for banners, titles have that. After 60 days you need to bid again on a banner slot (best is to bid on your existing one since it matches expiration date perfectly and you avoid being without one).
The technical limitation is 147 banners, right now 123 are taken. The reason why the available ones are not offered immediately is that we want to distribute them evenly across the 60 days. It's a bit off right now because some time ago we doubled the period from 30 to 60 days, that's why you sometimes see a lot of them and sometimes very few. Should fix itself slowly over time.