Fowler's Modern English Usage. It's a reasonably well known 'myth'. I used to be in a lot of book groups and forums where there would be the most ridiculous pedantry and they worshipped the book. Always found it a bit funny that despite this whenever it came to this very well known example, people seemed to ignore what the book had to say about it

. Not anyone here, but I've encountered it a lot. I know which I prefer, but to rigidly insist one is definitely correct even after extensive discussions is a step too far to me.
I'm not sure what you mean by definition items are numbered. I think you mean by the convention of 'counting' things. Sure, it may not be a bad convention, but there's no meat to it once you get into pedantry. For example, what makes people think that items in a list or set have to be numbered? I think a lot of this has to do with people's misconceptions about 'numbers', 'variables', 'physical quantities' which goes into angelfishsolo's post about data, that only holds true if you see the world through a very narrow definition of what a few statisticians and social scientists think.
I'm very confused what you mean by 'volume' and 'unit'. This again seems to be a very loose convention that may be helpful rather than anything substantial.