Hi,
You can refer to this document for the complete configuration that you would need:-
http://www.cisco.com/c/en/us/td/docs/security/asa/asa80/configuration/guide/conf_gd/protect.html
You need to use the "per-client-max n" value set to 500
Refer:-
http://www.cisco.com/c/en/us/td/docs/security/asa/asa-command-reference/S/cmdref3/s1.html#pgfId-1627430
Example configuration:-
hostname(config)# class-map CONNS
hostname(config-cmap)# match any
hostname(config-cmap)# policy-map CONNS
hostname(config-pmap)# class CONNS
hostname(config-pmap-c)# set connection per-client-max 500
hostname(config-pmap-c)# service-policy CONNS interface outside
NOTE:- Match the specific traffic that you want to match this limit restriction onto.
Thanks and Regards,
Vibhor Amrodia