I have question here: is creating CASArray in Exchange server important?
If we we don't create CASArray what will be the effect on a server with only one domain and with 20 email accounts.
Unless you have multiple CAS servers in a single site, you don't need to create an array. You wouldn't need multiple CAS servers to handle the load of 20 users. The only other reason to have multiple CAS servers in one site would be to provide high availability. If that's the case, you also need either dedicated CAS servers (no hub transport roles assigned) or a hardware load balancer in order to get the benefit of a CAS array.