I always use 'whom' when it is governed by a preposition, as in your sentence.
I always use 'who/whom' when referring to people, not 'that' (just as we don't refer to a person as an 'it' unless showing utter contempt for the person.)
When the noun is countable, it is :
a small number of friends
Use 'amount' for non-count nouns:
a small amount of sugar added to a glass of champagne makes...
Mr.Matthews:
I am aware that you take a "descriptive" approach to the English language; but are you suggesting that what you call the "prescriptive" approach has no place in this forum? I am led to that inference, since I find your statement - nay, admonishment - that "It is not helpful to confuse or mislead questioners with such shibboleths" to be presumptuously imperious. One could be forgiven for even inferring from your tone of writing that you regard this forum as your domain!
I will continue to provide the 'correct' 'Queen's English' grammatical perspective, if for no other reason than to show non-native speakers how other - dare I say 'educated'; dare I say 'good' writers may express themselves.