The short answer is three. Three things is enough to make real progress, and few enough that you can actually finish them. Here's why three works, how to choose them, and what to do with everything else.
Why three works
When you look at a long list, every item asks for your attention at the same time. That makes it hard to start anything, and easy to spend the day on small, easy tasks while the important one waits. With three things, you can see the whole day at once. You know what matters, and you know when you've finished.
Finishing matters more than most people think. Ending the day with everything done feels very different from ending it with half a list left. That good feeling makes it easier to start again tomorrow.
How to pick your three
Start with the thing you're dreading. It's usually the one that matters most, and it gets heavier every day you leave it. Then add anything with a real deadline today. If there's still room, add one thing that moves you towards a bigger goal, like the next small step on a project you care about.
Not sure which to pick? Ask yourself which task you'd be most relieved to have done by tonight. That's your first one.
Make each one small enough to finish
"Write the report" might take a week. "Write the first page of the report" fits in a morning. Each of your three should be something you could finish today. If it isn't, write down just the part you can do today.
What to do with everything else
Keep a separate list for later. Writing a task down means you won't forget it, so you don't have to carry it around in your head. Each morning, look at the later list and choose today's three from it.
Keep things that are waiting on other people on their own list too. You can't finish those today, however hard you try. What you can do is send a follow-up, and that can be one of your three.
What if you finish by lunchtime?
Then the day is done, and that's a good day. If you have energy left and want to do more, pick one more thing from your later list. Just don't start the day with more than three, because the point is to finish what you start.