Your keyword limit is determined by your plan: 

This is the total amount of keywords you may add across your Rank Tracker projects.  

A single keyword tracked across several locations counts as several keywords.

For example, you are tracking the keyword "best backlink checker" for USA and UK, this counts as 2 keywords from your tracked keywords limit. We do not limit the number of locations you can add, only the number of tracked keywords.

*Note that even if your plan’s limit is more than 10,000 keywords, each single project can only track up to 10,000 keywords.

To check your subscription limits, you can go Account Settings=>Limits&Usage: 

If you need to purchase more tracked keywords without upgrading your subscription you can do it with by adding the desired number of keywords and a pay-as-you-go bill will be added to your next month charge. You can also contact support via live chat or support@ahrefs.com if you want to add more keywords on prepaid basis.