What is parameter to check handling connection server group?

Unfortunately v4.x iRules do not allow to do what you want (which makes further discussion inappropriate for this forum). However, you may be able to achieve the desired behavior by combining pool member priority and per node connection limits. A sample configuration:

pool priority_groups {
   min_active_members 1
   member 1.1.1.1:80 priority 10
   member 1.1.1.2:80 priority 10
   member 2.2.2.1:80
   member 2.2.2.2:80
}
node 1.1.1.1:80 limit 100
node 1.1.1.2:80 limit 100

In the above configuration, there must be at least 200 simultaneous connections to servers 1.1.1.1 and 1.1.1.2 before any connection is loadbalanced to server 2.2.2.1 and 2.2.2.2. Note that connection limits are global, which means that if servers 1.1.1.1 and 1.1.1.2 appear in other pools, connections received via those pools will be counted against the limit.