Hello Marco,
That is because the FWSM does have a limit, I have not seen any limit on the ASA, The asa does support way way way more than the FWSM, I have not seen any limit yet but I have heard that it will let you know as soon as is full of ACL's or you will start seeing a degradation of the performance. Anyway dude you have an 5585, that is a giant and amazing box You are more than safe.
Hope this helps
Julio
Julio Carvajal
Senior Network Security and Core Specialist
CCIE #42930, 2xCCNP, JNCIP-SEC