Many items are stackable which means they are stacked together to take up one spot in your inventory. This is accomplished by left clicking on an item in your inventory the click on another item with the same name and quality level.
Stackable items are also splittable in case you want to share some with another player or only use one item in tradeskill. Click left ctrl + left mouse click to split off one item. To split off multiple items, press left ctrl and left mouse click on the stacked item and drag to the right. You will see the number one appear on the bottom has of the item. Continue dragging to the right until you get the quantity you want. To decrease the quantity drag to left. When you reach the quantity you want, release the left mouse click. Mouse the mouse cursor to were you want the item and left click.