Have you set up the max-conn or emb-limit options when you created the static nat ..? if you did .. please see below :
Specifies the maximum number of embryonic connections per host. An embryonic
connection is a connection request that has not finished the necessary handshake
between source and destination. Set a small value for slower systems, and a higher
value for faster systems. The default is 0, which means unlimited embryonic
connections.
max_conns Specifies the maximum number of simultaneous TCP and UDP connections for the
entire subnet. The default is 0, which means unlimited connections. (Idle
connections are closed after the idle timeout specified by the timeout conn
command.)
Note This option does not apply to outside NAT. The firewall only tracks
connections from a higher security interface to a lower security interface. If
you set max_conns for outside NAT, the max_conns option is ignored.