By default, the Start Menu has a 3-column setup that can accommodate 3 medium size of tiles. But the latest build 10586, aka the Threshold 2 released as Windows 10’s first major update, has added a forth column of medium-sized tiles, making more rooms for those who heavily utilize the new Start menu.
To enable, go to Settings app → Personalization → Start, and slide on the option Show more tiles.
Here is the screenshot of the Start Menu with 3-column medium-size tiles.
And now take look the bigger Start Menu with 4-column tiles.