It depends on the day. As a teacher, I write a lot of tasks in my to-do list. I prioritize my items, and then I start with the most important and work my way down. I don't always get to everything, but at least the most important tasks get done. The rest can wait for the next day or the weekend.
At most, seven. Any more and I feel overwhelmed but I do keep a list of less immediate things to do on days that aren't too busy.