To make a package use a configured Multi-Package Limit and respect it, open the package in question, scroll down to the "Divisions" section, and select the appropriate Multi-Package Limit in the dropdown.  

Click on the Save button at the bottom of the page to save your package. 

It's important to note that the Multi-Package Limit does not replace the individual package's limits if any are configured. All limits are respected. For example, let's say you configured a Multi-Package Limit to be 20 and one of the packages itself is configured with a limit of 5, the limit of 5 will be respected for that package.

Also, it is the same if your package is configured with a limit of 100 and it is configured to use a Multi-Package Limit of 20, the limit of 20 will be respected. That package would not reach anything higher than 20.