Limit Traffic on Virtual server & redirect it to "Busy" Web page

You don't need an iRule to implement this functionality. Priority Group Activation could be used with connection limit instead. Specify a connection limit on all your normal pool members and assign them all priority 10. Add the "apology server" (the one that sends the "Sorry, we're busy" message) as a pool member with priority 1. Set the Priority Group Activation to "Less than 1" member. If and when all the normal servers reach their connection limits, the apology server pool member will be activated and clients connecting to it will receive the "Sorry" message. When one of the normal members once again is below its connection limit, it will start accepting new connections, and the apology server will be "deactivated" until it is needed again. ("Deactivated" meaning it's there, but won't be used for load balancing decisions.) See https://devcentral.f5.com/questions/priority-group-activation-why-use-it