Just 1. If you point more than 1 you get penalized for duplicate content.
		
		
	 
Depends on how you are serving it.  If you are doing a 301 redirect it should not show as duplicate content since you are telling all inbound traffic to go to the authoritative/canonical domain.  If a search engine is ignoring that, then that is on the search engine and not the site.
Now, if you have the site answering on multiple domains as what is "called" or using a 302 redirect... then yes, duplicate content can be an issue.
I have seen several times that people have the 
.COM TLD answering, then you use the 
.ORG/.NET TLD and it takes you to the same site, but maintains the 
.NET/.ORG TLD in the address bar.  But honestly.. is it any different than really having your site answer on both 
WWW and non-WWW visits instead of redirecting to one or the other? It just happens that most engines deal with that directly now.
There are a wealth of sites that redirect the 
.ORG/.NET/.WHATEVER to a main 
.COM domain to keep folks from poaching as much as possible on the most common TLD's.
 
   
Now, the question becomes do you really want to redirect other domains to  yours?
If you have/buy a domain dealing with race cars, do you necessarily want to redirect it to one that deals with restoring antique cars?  You can also have the issue of bad "juice" or poor reputation for the other domain.  If it is one that has been known to have trash content, do you want it redirected to your current domain?  Or do you want to just have it redirect certain inbound links to another site?