The option for the thread count for SCM polling is only shown when there are more than 10 items in Jenkins.

This can have fatal (for Jenkins) consequences:

• Folders support: The option is not shown if you have e.g. ~5 folders with 4k items each, a situation that likely warrants a sane limit to the number of parallel polling operations.
• The option is hidden when the number of jobs drops below 10, and any form submit results in the default 0 (no thread limit) being set.

Both combined can quickly kill large Jenkins instances.