DNS zones limit doesn't apply to DNS Wizard
A client can also create DNS zones with the DNS Wizard if he already reached his DNS zones limit and the default interface already throws an "The max. number of DNS zones for your account is reached." error.
Further testing showed that the DNS zones created by the DNS Wizard wouldn't count to the DNS zones limit. For example if the DNS zones limit is 1 the client can create unlimited DNS zones using the DNS Wizard and exactly 1 DNS zone using the default DNS zones interface. The order of zone creation using the DNS Wizard and default interface doesn't matter.
I have taken a look at the database and noticed that the SOA records created by the DNS Wizard are owned by the admin group. Those created by the standard interface are owned by the client's group.