That's like asking how many windows should you have in your house.

It depends on a lot of different things, your niche, your traffic, your audience, your layout, the ads you get - what works for one person doesn't necessarily work for you. You need to experiment.

how many ads unit i should use for a page ?

Your page needs to look natural and not overcrowded, too many ads will turn people off since they will feel there is nothing else to the site.

Is your site fixed-width or fluid, your pages long or short, custom-built, cms or a blog...many different possibilities for different set-ups.