If you enter a value for the Number Of Results parameter that is larger than the number of features in the Candidate Features layer, the tool will reset the Number Of Results to a valid entry. If you select MOST_SIMILAR or LEAST_SIMILAR, the Number Of Results parameter will be reset to match the number of candidate features. If you select BOTH, the parameter will reset the Number of Results to half the number of candidate features.
If you weren't expecting this warning, check to see if you have an unexpected active selection associated with your Candidate Features.
If you don't remember the exact number of features in your Candidate Features layer, you can enter any large value (9999999, for example, as long as the value is larger than the number of candidates) to indicate you want to rank all Candidate Features.