The limit is currently set to 10K keywords that can be monitored in total across all marketplaces.
This is a necessary security measure to ensure that customers cannot "break" our crawling system by adding several thousands of keywords and thus slow down the tool or even bring it down for all other customers.
This limit is generally not reached very easily though and should be far too many keywords to oversee / manage anyway.
Recommendation: Only Relevant Keywords
Our advice is therefore to only track highly relevant keywords. Typcially there are rarely more than 50 Keywords that actually lead to sales (visible via PPC).
Using this kind of limitation will also ensure that your Cockpit's Ranking Index becomes far more meaningful, as is it will actually reflect your organic sales movement.
Also, please note: for the "Optimize Keywords" feature, keywords do not need to be moitored, but merely added to the respective product.