Keep in mind that LDAP maximum is set to 500 in OpenLDAP config. You can change that if you need to.
If you need more users, edit
/etc/ldap/slapd.d/cn=config/olcDatabase={-1}frontend.ldif
and change olcSizeLimit: 500 to an amount of users you need. In fact it is not 500 users, but 500 entries. Computeraccounts are also part of those 500 limit.
BTW this is not a Zentyal standard, but an OpenLDAP default:
5.2.5.6. olcSizeLimit: <integer>
This directive specifies the maximum number of entries to return from a search operation.
Default:
olcSizeLimit: 500
See the Limits section of this guide and slapd-config(5) for more details.
See
http://www.openldap.org/doc/admin24/limits.html