The characteristics of a sub-woofer is that the low frequencies spread out over a wide area and generally have no noticable point of reference. I have not seen the need for more than one sub if you are just playing a keyboard.
Powered or un-powered. That generally depends on what you have for existing equipment. If you do not have an amp capable of driving a sub then generally a powered unit is the best approach. If you buy an external amp make sure it has the power to drive the sub plus other speakers.
Dick