Many of the wonders and buildings you can place Great Works into have slots for more than one work. In such buildings, there is an optional Theming Bonus that will give you extra base tourism; usually +2 in a building with two slots, +3 in a building with three, and +4 in a building with four.

Each building/wonder with a theming bonus available has different requirements to earn it. Sometimes, you need to have all the works be from the same civilization and era, sometimes all from different civilizations and eras, sometimes it may be something as complex as "Must have two Great Works of Art and two Artifacts, all of which are from different eras and civilizations".

You can see all your theming bonuses from the new Culture Overview screen, in the "Your Culture" page which lists all your Great Works and their locations. To see the requirements you'll need to meet for any building that has more than one slot, hover over the large number to the right of the slots; that number is your current theming bonus, and hovering over it should bring up a tooltip with the details needed to get the bonus.

A quick browse through the Civilopedia lists the following as having more than one slot: