You only need one. You can't really use more than one at a time, so if you are working on your own, one is just fine. I've had one for ten years, and since I don't own more than one sledgehammer, having two doesn't suit me.  Now, some people argue that having two is good just in case because sometimes your splitting wedge gets lodged in a log and won't come out. Alternatively, if you are splitting a particularly large section, you can line up more than one wedge and then leapfrog as you hit each one in turn.  If you prefer, you can invest in things like diamond-splitting wedges and traditional wedges for knotty wood, long logs, or rounds for firewood.