Just to elaborate on the hotkey method that Laurie mentioned... In Blender 2.8, holding CTRL while pressing a number on the keyboard will automatically subdivide that selected object with whatever number of iterations corresponding to the number on the keyboard you pressed. It does this by instantly adding a subdivision surface modifier to the selected object, and changing the viewport iterations to that number. So, let's say you select the default cube, hold CTRL and press the number 4 on your keyboard; a subdivision surfaces modifier will be automatically added, and the iterations will be set to 4. This is why it's important to start with a lower number like 2 or 3, because higher iterations can crash certain systems that aren't quite powerful enough to handle a lot of polygons.

Also, if you choose to add subdivisions to an object with shortcuts, remember that you need to use the keyboard numbers, not the Numpad numbers. Holding CTRL and pressing the numbers on your keyboard Numpad (if you have one) will change the view perspective or camera view.

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