'Who' is a subject, 'whom' is an object. Easy way to know when to use 'who' versus 'whom:' replace with 'he' or 'him;' if the former works, it's 'who' and if it's the latter it's 'whom.'

"For whom the bell tolls" becomes "For him the bell tolls" (makes sense) while "For he the bell tolls" makes no sense.

"She gave the letter to who?" is incorrectly "She gave the letter to he" whereas "She gave the letter to whom?" becomes "She gave the letter to him?"

And remember "a number" is plural, while "the number" is singular.

Anybody wanna share some more cool off-the-cuff grammar rules?

(A random post, I know. This journal has been devoid of posts of real value lately. Here is my contribution for the month of April).