Keywords limits is the maximum number of keywords to track in your account. For each keyword tracked in a search engine, it will take up 1 keyword credit. To manage how keyword limits are allocated to Campaigns in the account, navigate to the Campaigns page in the Account Settings section of the left navigation :
At the Campaigns page, you can click on the wording (Either a number of (no limit)) under the Keyword Limit column for the campaign you'd like to adjust:
You will get a popup, allowing you to define a new keyword limit, after entering the number click on the tick icon to save.
You may remove a keyword limit by leaving its value blank.
You can also change keyword limits in the campaign by modifying campaign settings.