Your keyword limit is determined by your plan:
Please, mind this is total amount of keywords you may add across your Rank Tracker projects.
A single keyword tracked within several locations, counts as several keywords.
For example, you are tracking keyword "best backlink checker" for USA and UK.
This will count as 2 keywords from your tracked keywords limit. We do not limit amount of locations you may add, just amount of tracked keywords.
If your total tracked keywords exceeds this limit, adding keywords/locations will be disabled.
To check your subscription limits, you can go "Subscription plan":