In English, the words 'number' and 'count' can both act as nouns or verbs, but it's probably more common to see 'number' used as a noun, and 'count' as a verb. So you could argue that 'the number of foos' or 'num_foo' sounds more familiar than 'the foo count' or 'foo_count'. It certainly sounds more natural to me when referencing a quantity that isn't constantly changing. The word 'count', even when used as a noun, suggests to me a value that is going up over time.
Ruby and Python have .count methods, which demonstrate the word being used as a verb, rather than a noun. In Ruby you might say:
foos.count # Count how many elements in the array 'foos'
Still, this returns a value representing the number of foos, which is exactly what you might expect if you just referenced a variable called 'foo_count'. So in some ways, the fact that 'foos.count' and 'foo_count' look similar is kind of nice.
'Number' can be ambiguous in some instances, since it's common to store numbers that don't represent a quantity of something. Other people have mentioned IDs and credit card numbers already. Here's another example:
num_string
Looking at that variable name, could you guess what it represents? Is it an integer representing the quantity of strings, or is it a string representation of a number?
So I'm just thinking out loud really, and giving some pros and cons for each as I see them. The reason I'm even on this old page is because I find myself using the two inconsistently and thought I'd see what other people are doing.
BTW, I don't like 'nr_foo', as 'nr' really doesn't suggest or sound like the word 'number' to me at all. It sounds like 'ner', or perhaps stands for 'not rated' or 'national rugby'. :-) And I won't even venture to say what fooCnt sounds like. Just no.