Hi Julio,
The "external" queue is used for special workflow steps which are built as external steps. External steps are typically used in situations where you are contacting an external system and polling for results. For example the "InDesign Media Extraction Process" step is implemented as an external process. The workflow engine uses the external queue for processing the polling. You can learn more about external processes here [0]
The non external queue is used for processing other workflows so you could change the value here to max threads. However in your situation you have another option - you can create a queue for individual workflows. From the page [1] (after you have run a dam update asset workflow since the machine has been turned on) you can see topic statistics for each topic. In your case you will see the topic [2] and [3] and [4] which are all used in the Dam Update Asset workflow. Create queues for each of these topics and give them appropriate values that meet your needs. The trick here is that generic queues are added to the system that cover all workflows in the same queue and the topic for each job is matched with wildcards, but you can specify more specific topics in additional queues to further customize the configuration for particular workflows.
Hope this helps,
Will
[0] http://dev.day.com/docs/en/cq/current/javadoc/com/day/cq/workflow/exec/WorkflowExternalProcess.html
[1] http://localhost:4502/system/console/slingevent
[2] com/adobe/granite/workflow/job/etc/workflow/models/dam/update_asset/jcr_content/model
[3] com/adobe/granite/workflow/external/job/etc/workflow/models/dam/update_asset/jcr_content/model
[4] com/adobe/granite/workflow/job/etc/workflow/models/dam-xmp-writeback/jcr_content/model