When we want to express the quantity of something (countable), we may say "The number of something is + number", like "The number of books is 3."
Can I say “The quantity of books is 3”? I mean "The quantity of something is a number". Is that correct?
If the stuff is uncountable, like water, can I say "The amount of water is 3 cubic meters"? So that the pattern becomes "The amount of something is + number + measurement units". Is that the formal usage?