Most nouns are countable nouns, which means that they refer to things that can be enumerated (texts, computers, shoes, rockets, etc.). These nouns take comparative words like several, many and fewer.

I have several computers, but I have fewer than Erik, who has many more computers than I do.

Many nouns are considered uncountable, which means they refer to things that cannot be easily enumerated (milk, sand, memory, space, power, access, information, etc.). These nouns take comparative words like much and less.

My new phone has much more memory than my old phone, but it also has less screen space.