Next to the number of Artboards are four icons and an arrow. The four icons give you options to arrange your Artboards. I usually leave it on the first option, which will arrange them from left to right and add rows going down. I also recommend adding to the Spacing (area between Artboards).
When you choose the Artboard Tool, the current Artboard gets a bold dashed line around it.
Drag the handles to resize the Artboard, select the Trash Can on the options (at the top of your screen) to delete an Artboard. There are icons to add new or rename Artboards, change x and y positioning, and adjust width and height, among others. Hover over the icon to learn its function.