A flightaware search shows only 10 active 5500 series aircraft --

N519US, N521US, N522US, N523US, N525US, N530US, N531US, N532US, N533US, N534US

I saw a report that there were 22 5500's at the time of the merger. Here's 10 other aircraft that had hits on flightaware with their final flight date --

N501US - 04/13/13
N502US - 12/19/12
N503US - 05/04/13
N507US - 09/30/13
N517US - 07/06/10
N518US - 07/27/10
N520US - 07/31/10
N527US - 08/15/11
N528US - 09/06/11
N529US - 08/21/10