IWe started taking delivery of the 717's around Sep of 13 give or take one month. We get 3 per month until all 88 are delivered. We should have about 48 to 51 onboard. That leaves around 36 to go. Keep in mind that the slots for aircraft coming the next few months have already been bid. 88 aircraft should easily support 4 bases. We had about the same amount of 737's prior to the merger with 5 bases.
Edit: there were 49 aircraft onboard as of The end of NOV. Deliveries do appear to be accelerated a little bit to 3 to 4 per month. Last delivery Dec of 15. At that point the A321 start arriving so narrow bodies should continue to increase.