How "tracked keywords" is calculated: 

No. of keywords X No. of locations = Total tracked keywords

For example:

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": 

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