At least once. For me?
But, before you go, pin this list so you're sure to at least have the basics!
1) Cellphone and Good Charger
You're going to use your phone A LOT. It's easy to use for pictures (I even use mine to take pictures of things like where I parked or merchandise that catches my eye), you'll use it to get in touch with any friends attending the convention (I even made new ones!), and you can make notes if you're at an awesome panel and want to look something up later.
I say GOOD charger because I screwed up and brought my subpar one. I have a "rapid charger" that pumps my phone so full of juice it gets hot on occasion (probably not good for it but it came in the box so....) and one that didn't originally come with the phone but "fits" so I use it for longer, overnight charges.
You want the best charge when you're sitting between panels trying to bump your phone up to functional.
2) Food and Water
"Don't they have food and drink at these things?"
Yes. Yes they do. But it's convention food...think $10 for that crappy cheeseburger you got in high school. My husband figured out that the drinks in the vending machines were only $2.50 each...as opposed to $4 or more from a vendor booth.
So pack some hearty snacks and bring a bottle you can fill at water fountains! Save yourself the money.
Speaking of money...
I don't like carrying cash around. I've always hated doing it, even before I got my card. But a lot of vendors only take cash and it's much faster to hand a $10 bill instead of going through the song and dance of swiping a card. Plus, it's not uncommon to run into technical issues with card readers and every second counts when you're running to your next panel.
If you plan on going to the big celebrity spotlight panels, I suggest showing up early.
Really early if possible.
Like...an hour early if you really want good seats.
Sitting in a single seat for an hour is really boring. It's double boring if you forget to bring something to read. (I speak from experience)
5) Repair Kit
Now, my Comicon has a "repair center" stocked with all sorts of tape, glue, etc... but I don't think every convention is like that. (Correct me if I'm wrong in the comments!)
What you put in your repair kit is dependent on your cosplay. I had tights so I put clear nail polish in mine. I also had eyelash glue (for errant edges of falsies) and makeup wipes (for quick cleanup of lipstick reapplication) You may have a tube of super glue or safety pins.
Basically, look at your costume and ask "What can go wrong?" You may have a smooth convention with no issues...or you may tear your pants brushing by someone. Be prepared.