Tools with a similar function are grouped together by default. To see the other icons in a group move your mouse over an icon. Depending on a preferences setting (see below) you can see the other icons in a group by just hovering, or after clicking on the icon. The little triangle in the bottom right corner of tool icons tells us that there are more icons in this group.

The Toolbox preferences has an option to disable grouping, but also to change the groups or make new ones.