What to say if I want to tell approximate number of something. What should I say? E.g.
I have (around/ about/ some) 5 books.
Is there a rule for number approximation?
There are some alternatives, depending on the context, situation and use. I provided an example for each of them (taken from the NOAD):
So, in your case, I'd use approximately, about or something like (choose accordingly to your situation, i.e. formal vs informal, written vs oral, etc.)
Purely commentary on what others have written:
If you're speaking informally, about and around are the best: - I have around 5 books. - I have about 5 books. (I can't think of any contexts to distinguish them; they feel pretty similar)
Roughly works too by it feels like the answer was meant o give an exact number but you just don't know.
Some has a jaunty (and therefore not common) feel. "There are some 500 live cobra in the with the stolen bank vault, said the henchman"
Approximately is more technical sounding (a more formal version of 'roughly').
Something like is nicely informal (colloquial and common)
Circa is very academic sounding and usually used with dates (anything else is very formal sounding).
Edit: I just heard on national news an announcer say "some 300 deaths", so I think our impression that 'some' is informal is totally wrong. It's not overly formal, just acceptable in news.