If you insert a variable ($client_id) in place of account, then the XPub key will change all the time. To keep XPub unchanged, you can change only the last two fields (change and address_index).
It all depends on which branch's xpub you are taking to derive addresses. For example, you can use xpub of
m / 44' / 0' branch to derive as many accounts as you want, or you can use the master public key to derive all public keys from all branches.
The chief point is that you
must use a different account for each customer if you want to keep them completely separated from each other.