According to usage rules, fewer is only to be used when discussing countable things, while less is used for singular
When should Fewer be used?
'Fewer' and 'Less'
Generally, fewer is used when the number of things is counted ("fewer problems") whereas less is used when the number is measured ("less trouble" or "less time").
How is fewer used in a sentence?
1 Women commit fewer crimes than men. 2 There seem to be fewer tourists around this year. 3 The detective discovered no fewer than 35 fingerprints. 4 No fewer than 10 students were absent through illness.