Boards with number pads on the left are called south paws. I'm a lefty and I've always thought the numpad is in a more convenient location on the right, but what do I know.

It depends on how you use your mouse, I tend to keep one hand on the mouse and one on the keyboard for a lot of the day and it's frustrating not having space to swing left when there's a numpad. I've been using a tenkeyless and an external numpad for many years now and find it much preferable to a full-size keeb.