Hi Rodrigo,
I had a similar requirement, I don't use zabbix though, I use PRTG but the process was the same. You cannot query the director for the number of clients that are connected to each AP, you need to query each AP individually. I had created a custom OID sensor with the following value on each AP:
1.3.6.1.4.1.25053.1.1.6.1.1.1.4.1.3.21
This gave me the number of clients connected to that AP.