I like your second question better than your first....

But to answer your first, you can have up to three ad units, plus one link unit, plus a search box, plus referrals. But before you go wild, double check your plans via AdSense Help.

Now, as to your second question, the benefit would be to fill up a page that otherwise had little content. Most people with content find that fewer ads is better because the average click is dragged down by more ads....

Yes, the fewer ads the better..

Until you removed that last one... but then you'll be able to advertise your site in Adwords to get a lot of visitors and have many deductions for your tax return.

If you hav a very large page with lots of content, you may benefit by including more Adblocks (up to the limits) by ensuring ads are on screen more often as your visitor scrolls down.

It may, however, be better to split the page.