There is a lot that could be said here, but the basic problem is that Chinese just doesn't have articles, either definite or indefinite. Chinese is not the only language like this. Japanese and Latin don't have articles either.
Here is the difference between English and Chinese in its simplest form:
- He is student. Not possible.
He is a student. Possible
- 他是一個學生。 Possible
Is it possible to produce a BAD sentence like (1) in Chinese? No. So we can't really say that in Chinese sometimes the article is dropped. There is no article, so nothing has been dropped.
This is very confusing to English speakers. My solution is to give up. Don't ask why, just accept it, and don't use 一個 as a substitute for 'a'. Using your example:
- I want an ice-cream. (vague)
- I want one ice-cream (precise)
I don't think vague/precise captures all of the differences here, but let's be pragmatic:
- is what you say to a friend to suggest that you go into the ice-shop and buy something.
- is what you say to the guy at the counter.
How does this work in Chinese?
If you were to say 5' to the clerk, it would be a bit odd, I think. The clerk mumbles under his breath 廢話! 6' produces the desired result.
If you were to say 6' to your friend, not odd, perhaps, but on a bad day it is possible your friend would think you were trying to buy an ice cream from her (跟我要沒有用), or telling her to go get you one (自己去買吧!)
Pragmatics is thus very important in interpreting the lack or provision of a numeral in front of a noun. That's all there's room to write here.