yes on simes 2 you can add up to 3 floors! but on sime 1 im not so sure, take a look in the manual or go on the website!
if you google it that may tell you what you need to know
On Sims one you can have an upstairs and a downstairs. Just click on the building tool and then the button that looks like stairs. With Sims 2 I think you can have as many as you want cos there is a house already made for you in strange town that has loads of stairs.....