Q: How many keywords can be created in Sitelinkx Pro?

A: Simple answer - as many as you like *

* While you can create as many keywords as you like, please keep in mind that creating a huge number of keywords that should be replaced might slow down your site.
This depends of course massively on the server / hosting package you are using - small servers / hosting packages (read: cheap hardware) normally don't have much available resources in terms of RAM, processor power and HDD speed, which can massively impact your site's performance.

This is only natural - the more keywords must be replaced, the more computing power is needed.
If you are creating several thousand keywords with 49-cent-per-month hosting packages (as some people are doing) , you don't have to wonder if your site gets slow.

Or put another way, an ant cannot carry an elephant