For Cisco Jabber release 11.8 and later, there is a Disable_IM_History parameter for jabber-config.xml.
I don't think there is any mention of how many chat windows you can keep opened in any Jabber related documentation.
I tried on my machine with 25 chat windows open without any issues. I am using Jabber Version 220.127.116.11081 Build 301081. I will never open that many windows at a time but had no issues opening it.