Here is the rule:
© 2010 Ugur Akinci
FEWER is used for objects that you can COUNT one by one. LESS is used for objects that you can only MEASURE in BULK, and cannot count individually.
For example, you can count the members of a TEAM but you cannot count the RAINDROPS when it’s raining.
That’s why you say:
“Team A has FEWER members than Team B” but “It rained LESS on Wednesday than it did on Tuesday.”
“The dispatcher had fewer calls this week.”
“The new filter allows less pollutants into the assembly room.”
“We need to use less money to launch our new product.”
“Fewer error messages indicate an improved software, correct?”