It's doncumented in the "Configuring Health Monitoring" of the configuration guide "server load balancing".
For ACE4710 running A3 :
The ACE supports 1000 unique probe configurations, which includes ICMP, TCP, HTTP, and other predefined health probes. The ACE can execute only up to 200 concurrent script probes at a time. The ACE also allows the opening of 2048 sockets simultaneously.
You can associate the same probe with multiple real servers or server farms. Each time that you use the same probe again, the ACE counts it as another probe instance. You can allocate a maximum of 4000 probe instances.