"the number of" precedes plural countable nouns, as in:
"The number of paper products..."
"the amount of" precedes non-count nouns, as in:
"The amount of oil..."
How if we want to say "the total" of both countable and uncountable nouns?
a. The number of paper products and oil is hard to measure.
b. The amount of paper products and oil is hard to measure.