I want to know a share hosting (eg,dreamhost or hostgator's share host ) 's traffic upper limit. 20000-40000IP/day, ok?
There is really no way to quantify this. Most of it is dependent on the way you choose to deliver your content. A wordpress blog with tons of plugins will cap out much quicker than a static html page. If you are truly worried about reaching the upper limit, start with a VPS and upgrade from there.
Hostgator has a limit on how many processes you can have going at once. I think it is 25. This could limit the traffic you could get at once. The post above explains well how fast these 25 processes can be used up. They will lock your account if you exceed this limit from what I understand.