I would say at least 6, but I've seen some sites going with 8 or even 9 now....but for our link directory, I don't have any rule of submission regarding how many recips, as long as it doesn't look like a huge banner farm....you said you feel 12 looks cluttered...my suggestion is if you feel this way, don't do 12 then. Remember, its "your" site....not a LL's or directory's site. Although you still want to comply with the basic submisison rules of them.
Hope to see more of your sites over with us
Cheers