By default all LDAP contexts created by Confluence use a connection pool with a minimum size of 10 connections, and no timeout on the connections.

Contexts are created for general queries, using the credentials specified in atlassian-user.xml. These all use the same pool.

A context is also created when a user authenticates. These each use their own pool, as they are using different credentials.

So a pool with 10 connections, which will never be closed, is created each time a user logs in.

This is fixed in 2.3.2 by setting <initSize> to 1 and <timeout> to 30000 (30 seconds) and authentication connections don't use a pool at all.

In 2.3.1 and earlier you should add
<initSize>1</initSize>
<timeout>1000</timeout>

to reduce the number of outstanding connections.