Depends on a lot of things, but here's a particularly good day of sustained traffic in the top 10. https://img.skitch.com/20121001-earf66yyjrpj7b11tjek89i9ii.j...

The source of that spike was 300+ concurrent uniques high turnover for a good part of the day.

I submitted a few "interesting" questions from StackOverflow last year (by interesting I mean they had interesting subjects, but weren't link-baity; my poor memory has forgotten what they were though!). IIRC, both of them got about 200-250 votes (on HN) and about 20-30 thousand visits. How do I know? I submitted using the share link StackOverflow gives you (it's something like stackoverflow.com/question/1000/2000, which means that user 2000 (me) has linked to question 1000), and about the time they reached 100-120 votes, SO awarded me a badge along the lines of "you shared a link that 10,000 unique IPs have visited".

So, based on my very limited experience: unique visits are about 100 times the number of votes on HN.

#1 on HN for almost a full day: ~21,000 visits and at peak 815 people with the site open at once.