There is a maximum limit of 1000 keywords per campaign
in order to ensure efficient report generation and convenient viewing. Adding more than 1000 keywords to a campaign will cause some functionality to be disabled.
If you need to track more than 1000 keywords for a site, then best practice is to create additional campaigns (e.g., separate them by keyword category, location, tags, etc.). If you have an Enterprise package, you can view rank performance data for multiple campaigns in the Portfolio Report
and by using cross-campaign metrics in the Insight Graph
and Single Metric Widgets