I think you can just insert a higher number into said box.
If that doesn't work...once you have made your thread, it will take you to your poll screen - where you customize your poll. There's a box under the entire poll, where you can modify the number of choices as well.
i'm a monster
i'm a maven
i know this world is changin'
never gave in
never gave up
i'm the only thing i'm afraid of