Fewer: Comparative degree Adjective and it denotes Number.
Less: Comparative degree Adjective but it denotes Quantity.
Example: Less visitors / came to see / Hawa Mahal this year. / No error
Explanation: In the above sentence, there is an error in the first part of the sentence as ‘less’ is a comparative degree adjective used to denote quantity; and number is denoted by comparative degree adjective ‘fewer’. So, ‘fewer’ should be used in place of ‘less’.
Hence the correct sentence would be:
Fewer visitors came to see Hawa Mahal this year.