Understanding Pixel Counts

Can anyone tell me how to compare pixel counts in terms of apples to apples?

I know that Olympus says that 12mp on a 4/3 sensor is more than enough. But how does this compare to full frame sensor ans pixel counts (like the newer canon and nikons that are out) and other DSL cameras. Nikon has introduced their d3200 with 24 mp.

How much of this matters? And at what point is the overkill as it relates to pixels?

Thanks in advance for your response and insights.