According to the FAQ-
How many tickets can I buy for each event?
Most events have a limit of two (2) tickets per person, but some events have different limits. For example, True Dungeon allows you to buy all the tickets for one event so you and all your friends can play at the same time.

How can I tell what the limit is for a particular event? It seems reasonable to assume that most 4 person events have the 2 ticket limit but what about a 6 person event? Or more? I would like to know this ahead of time before getting screwed over because I tried to buy too many tickets for a given event.
 

For most events, TD (and possibly a few others) being the exceptions, you can buy two tickets for yourself, and one for every person in your friends list, up to the limit for the event.  So, for an event allowing 6 people, you could buy all 6 tickets if you have 4 friends.  For TD, you can buy all 10 tickets for yourself.

As a follow up question- why would anyone want to buy 2 tickets for themselves when the system allows them to buy a ticket for a friend?

[This post has been removed]

It's also helpful if you have a friend that still isn't sure they're going to the con.  You can buy 4-day badges until they run out, usually in July, but if you don't get into the popular events when registration opens, you could be SOL.  This way, you can buy that ticket for your buddy, and then if he gets his badge later, you just give him the ticket (or transfer if an e-ticket).  If he never goes, you can return the ticket for system credit.

I use the “Another Ticket for Myself” option along with “Only Book If All Are Available” (or whatever it is called) on almost *everything* so that I can snag events for myself and my wife to participate in together without blocking her schedule.

We lightly discuss our wishlists while composing them and then we’ll sort it out on registration day when we know what exactly we got into. If I bought a ticket for her - and my wishlist was processed before hers - I could end up blocking an event in her wishlist that she would have preferred.

So, this way, I’m the hero not the villain - “Look, hon, I got us two tickets to XYZ event - come hang out with me since you didn’t get into ABC event.” Way better than “What?! You scheduled me into XYZ when I wanted to go to ABC?! What were you thinking - go drink alone!”

:)

It allows for more flexibility - Gen Con registration doesn't let you double book yourself for events.  If your "friend" thinks it would be fun to go to a 8 hour LARP from 11 AM to 7 PM Saturday, and they book a ticket for you, you will find yourself unable to buy tickets to events that overlap which that period (which is going to be almost everything on Saturday).  Be careful with buying for friends...