1. "A couple of" means "two."
I bought a couple of books this morning.
2. "Few" or "a few" means "three or more, but not many." "A few" is used in a positive way. "Few" is used in a negative way.
A few people watched the movie. (We thought nobody would watch the movie, but some people did.)
Few people watched the movie. (We thought a lot of people would watch the movie, but not many did.)
3. "Several " means "more than a few but still not many"; it describes a small group in a set.
Several trains were late yesterday.
4. "A number of" is similar to "several." It is sometimes used to mean "many," but not "most."
Last year, James visited a number of cities in Europe.
5. "Many" means "a big number."
The magazine contains many interesting articles.
6. "Most" means "a bigger group in a set; a majority."
Most athletes are very healthy.
7. "All" means "every member of a set."
All our employees have email accounts.