Troubleshooting the number of deployment threads
You can configure the number of threads that actually deploy content, as expressed by the Workers element in the configuration file.
The sum total of the values of the Workers attributes of the various Location elements in your Content Deployer configuration file represents the total number of deployment threads that can be active at any one time. This number should be somewhat higher than the number of CPU cores on your server. You can check the CPU load to tweak the ideal number of workers.
Note also that if you use a database pool of type tridion, the number of Workers must not be higher than your pool size (configured as the Size attribute in the Pool element of your Storage Layer configuration file, cd_storage_conf.xml).