You can restrict to use predefined label versus user-defined label but you can not restrict to use multiple labels from the predefined label list.
If you navigate to your board within ADMIN then >Features>Labels you can set the max number of labels to be added to a single idea.
Unfortunately, that option works on the idea page/forum page/ blog page but does not apply on post page's label list.
Here is the related idea with acknowledged status.
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