You can't use all of them with one account, it's written in the Terms Of Service (
http://aws.amazon.com/powerof60/terms/)
What I did was setting up different accounts (and a few from friends), used one code for each account, and that's it.
I mined several different altcoins and I ended up, using a total of 1.200kH/s (6 codes/miners at the same time) for 6 days ($0.65/h per instance) about 0.9 BTC when trading the altcoins myself on Cryptsy.
And obviously you can open for yourself, but they monitor IP addresses, companies and data you put in, so not that easy to get 20. As I said if anyone is interested let me know, these were selling on eBay at $29 to $50 each, and I'd give them away for less than that (just to repay the work I did to obtain successfully 20 of them), probably no more than 0.4 for all 20.
If not I'll use them myself lazily with hashco.ws and I'll get probably more than that but with a bit of work.