How to improve the concurrent num in ldap?

[marisu]

zimbra602 oss , redhat as 5.4,
I test the concurrent auth user all day, finally,i got a result, when the concurrent num reach 100, every things goes well , but when i set the num up to 200, i got lots of errors. I think maybe i should adjust the ldap config files let ldap can support more concurrent users auth at the same.
Does this config file(s) exist ? At where ?

[dalmate]

sorry, but what do you mean "concurrent users"? Because in my system, it always has about 500 users online at a time and we don't have any problems.