You might write about fewer challenges or less competition facing your business, but it would be incorrect to write about less challenges or fewer competition. Why is that and how do you decide which to use?
Less is used with things that can't be counted individually, such as efficiency, money, competition or time (uncountable nouns).
Fewer is used with things that can be counted individually, such as products, employees, challenges or days (countable nouns).
But there are exceptions. For example, you'd refer to a time period of less than six months, not fewer than six months, because it would be referring to a single period of six months, rather than six individual months.