Definitions of "a number of":
- Collins Dictionary at https://www.collinsdictionary.com/dictionary/english/a-number-of
an unspecified number of; several or many
- Merriam-Webster at https://www.merriam-webster.com/dictionary/a%20number%20of
more than two but fewer than many : several
- Oxford at https://www.oxfordlearnersdictionaries.com/definition/academic/number1?q=number
several people or things; some
According to Collins Dictionary, the phrase could possibly mean two or many -10 or even more for instance- while Merriam-Webster excludes both two and many.
What number would you imagine from the phrase "a number of"? Would it be somewhere around
- two to five,
- six to nine, or
- more than 10
It would probably depend on the context, but I feel that the phrase is often used in an ambiguous context like the below one.
On November 26th, a number of NSO Group's workers filed a lawsuit against Facebook, claiming that the social-media giant has unfairly blocked their private accounts. https://www.economist.com/business/2019/12/12/offering-software-for-snooping-to-governments-is-a-booming-business
How many workers approximately do you guess filed a lawsuit against Facebook?