Just one. I believe that it's silly to have more than one Alt account unless you're on staff and have a need for more than one account. For example to test something like permissions, or a new forum feature.
I also believe that it's not quite right that most people won't mention their mains on their alts, but that's really a minor thing, and sometimes it's better for them to hide under the alt so long as they're respectful of the rules. I.E. not sockpuppeting.
But of course, this is just me.