To configure the number of allowed threads used by the Channel Manager, change the value of org.jasig.portal.ChannelManager.threadPool_initialThreads in uportal-impl/src/main/resources/properties/portal.properties.
The max threads property in portal.properties is not ever used. The channel thread pool will always remain capped at the initialThreads value.
For example, to configure the thread pool to a maximum of 150 threads:
org.jasig.portal.ChannelManager.threadPool_initialThreads=150