Hi, Alexander!  Recently you wrote in a message to Ardith Hinton:   
      
   ak>  I can understand "police are a corrupt gang", for instance.   
   ak>  But in this case there is no article  before "police", and   
   ak>  we imply that "police" is uncountable (in British English).   
      
      
             According to my OXFORD CANADIAN DICTIONARY, it is usually treated as   
   plural.  There's more than one definition... but in the above example you seem   
   to be alluding to police *officers*.  Police officers are human beings & human   
   beings are countable.  I often hear in news reports that "Police are attending   
   the scene."  I take that to mean the number of individuals, while unspecified,   
   is greater than one & other individuals may be on their way.  The reporter has   
   deadlines to meet & some time will elapse before the story gets to us....  :-)   
      
      
      
   ak>  "A number" can express many people.   
      
      
             Uh-huh.  And by your own definition it's a countable noun if you can   
   put "a" (= "one") in front of it... a rule of thumb which FOWLER'S also cites.   
      
      
      
   ak>  For instance:   
   ak>  A number of casualties is too high.   
      
      
             You could say... without specifying an exact total... that "a number   
   of people" have died as a result of an earthquake, tidal wave, flood, etc.  In   
   such cases various people may be "missing & presumed dead" and/or the estimate   
   is revised when further information becomes available.  If the total is broken   
   down into categories (e.g. Japan, Haiti, USA) with subtotals for each it would   
   also be appropriate to use the plural verb form WRT the subtotals.  Either way   
   what I'm thinking of at this point is *recording actual or fictitious events*.   
      
      
      
   ak>  Do we use "is"?   
      
      
             As an *expression of opinion*, I'd probably say:   
      
      
                        "Any number of casualties is too high,"   
                                          or   
                        "The number of casualties is too high."   
      
      
   Strictly speaking, the prepositional phrase is just a modifier & the verb must   
   agree with the noun... but the above doesn't tug at my heart strings quite the   
   way [members of the community] do.  I think Rowling chose her words with care.   
   "A growing number of the xxx community" clearly involves more than one person.   
   All three groups mentioned in this sentence get the plural form of the verb...   
   i.e. a parallelism which, on the surface, may seem to indicate a level playing   
   field.  For those who are aware that the author had a choice WRT her treatment   
   of the second group, however, her choice is significant.  From Harry's POV the   
   second group are the "good guys".  If the reader doesn't understand right away   
   which side the author is on s/he'll figure it out eventually.  OTOH, those who   
   depend too much on rules don't see the power & beauty of British English.  ;-)   
      
      
      
   ak>  Well, maybe also we can say that "number" in "A growing   
   ak>  number believe that..." is used as a mass noun.   
      
      
             A collective noun is different from a mass noun.  We're not speaking   
   of different varieties of flour etc.  Since the number in question is growing,   
   the reader is in much the same position as if s/he heard "Police are attending   
   the scene."  By the time s/he gets the news the number may be inaccurate.  :-)   
      
      
             Here is another example, from another British author:   
      
                        ... all at once I saw a crowd,   
                        A host, of golden daffodils;   
      
                                       [...]   
      
                        They stretched in never-ending line   
                        Along the margin of a bay;   
                        Ten thousand saw I at a glance....   
      
                                            William Wordsworth, 1804   
      
      
      
      
   --- timEd/386 1.10.y2k+   
    * Origin: Wits' End, Vancouver CANADA (1:153/716)