Usually one so I can hear outside noises! If I'm on a phone call, running, or walking, both. I'm not aware if I have a preference for which side, though.
when i'm home i switch between my earbuds and headphones but it's usually earbuds unless i'm gaming. i'll wear my headphones when i'm gaming since my ears start to hurt after maybe 2-3 hours and that's when i know it's time to call it a day for games and give it a break.
when i'm outside though i'll use the earbuds (not the ones i have at home since those go in the ear, if that makes sense) that came with my iphone but normally only have it in one ear since i'm most likely on the subway and it's always safer to be aware of your surroundings. sometimes i'm with someone else (either my sister or a friend) so in that case i would have one with my music at a pretty low volume so that i can still chat with the other person when needed ♪(´▽`)
At school I always use both. That way, I can walk through the hallway in the most based way possible (which is obviously listening to Sonic and Mario osts) while not having to listen to what other people are saying. Usually at home, I only wear one in case my parents call me.
I only wear ear buds at work. I wear just one so I can switch to the other one when the first one dies. I don't wear them in public because it's v important to be 100% aware of your surroundings at all times.