How "tracked keywords" is calculated:
No. of keywords X No. of locations = Total tracked keywords
100 keywords x 1 location = 100 total tracked keywords
If you track those same keywords for 2 locations, it would look like this:
100 keywords x 2 locations = 200 total tracked keywords (in total)
Your keyword limit is determined by your plan:
If your total tracked keywords exceeds this limit, adding keywords/locations will be disabled.
To check your subscription limits, you can go "Subscription plan":