Can I have several wordpress instances running in a single Amazon EC2 instance?
Each one with a different URL.
Yes, you could for example achieve this with apache and a number of vhosts if you need to support multiple domains.
If you don't need to support multiple domains you can do it without the vhosts. Each wordpress-installation would have its own "directory".
To answer your actual question "Yup."
Beyond that... You'll want to look up Virtual Hosts in Apache (or some other web server). TiZon's answer explains also that you don't strictly have to run distinct virtual hosts.
Wordpress also includes "MU" functionality that lets you configure a single wordpress installation to have several sub-domains that are separate instances of Wordpress, but are all centrally managed.
So you could have something like:
And that'd all be run from a single instance of Wordpress.
For instructions on how to set up a Wordpress server, I kindly advise you to RTFM (click for link). The instructions there will be your best bet. Alternatively, contact me if you want me to just do it for you, for some kind of fee. (I run a bunch of stuff on EC2, including WP).