SharePoint products collect the enterprise keywords into a single, non-hierarchical term set. You can find this special term set, which is named the Keywords set, by using the Term Store Management Tool.
Software boundaries and limits for SharePoint 2013 gives a supported limit of 30,000 items in a term set.
What is the maximum number of keywords that can be used in SharePoint 2013?
Can this limit be modified?
As it turns out, there is no boundary or configuration setting for the number of keywords. It's just a recommendation.