Kindly Guide How to add Error 500 alongwith Existing Error 503 into the Irule

How about this, based on your current version:

  Don't send 503 or 500 errors out, try next server.
  After 10 consecutive failed attempts, send redirect

when HTTP_REQUEST priority 700 {
    set my_url [HTTP::host][HTTP::uri]
}

when HTTP_RESPONSE priority 700 {
    if { ( ( [HTTP::status] == 503 ) or ( [HTTP::status] == 500 ) ) && ( $count < 15 ) } {
        incr count
        HTTP::respond 302 Location "http://$my_url"
        event disable
    } elseif { ( ( [HTTP::status] == 503 ) or ( [HTTP::status] == 500 ) ) && ( $count == 15 ) } {
        set count 0
       log local0. "Sending to Main Page"
        HTTP::respond 302 Location "http://www.lenovo.com"
        event disable
    }
}

when HTTP_RESPONSE priority 800 {
    if { ( ( [HTTP::status] == 500 ) or ( [HTTP::status] == 500 ) ) && ( $count < 15 ) } {
        incr count
        HTTP::respond 302 Location "http://$my_url"
        event disable
    } elseif { ( ( [HTTP::status] == 500 ) or ( [HTTP::status] == 503 ) ) && ( $count == 15 ) } {
        set count 0
       log local0. "Sending to Main Page"
        HTTP::respond 302 Location "http://www.lenovo.com"
        event disable
    }
}