The ads that are showing in the lower part of your page will show ads with lower paying keywords. Too many ads will give you repeated ads anyway that are not very high paying.
you may try it. there is no penalty for that. Only 3 units will work for each kind (link units, block units)
If you put more adsense codes than it is stated in the TOS, they won't display and you are risking your account (this is not very possible, but possible).
I've seen some sites displaying more than three ads using iframes but, as others said, it is very risky as it is against TOS and may result in your account being banned. Your question is pretty much answered - you can put as many as you wish, but only three will work and you might lose your adsense account as a result.
You can only put 3 ads on your page. If you try to put more than that it will not display the other one. Also the CPC will be very low.