There are 17 Smithsonian museums and one Smithsonian zoo. You could do it all! If that's your priority, I would make a schedule based on location and try to get a few done each day.

However, there's a lot of other amazing things to do and see in DC, so I would suggest seeing the highlights of everything. For Smithsonian museums, I think you can't miss the Natural History Museum and African American History Museum. For other DC places, you should see the Lincoln and Washington Monuments, the original constitution, and the National Gallery of Art if you like museums. There's so much more to do and see, so I would consider your top priorities.