I think you still don't know what they talking about, you need some tutorial before you really splashing your money.
I don't mind give you a simple explanation here.
Not based on any statistic but my own observation, most of the mice in the market are Ambidextrous mice.
As mentioned, it can be used for both left hander and righties. The mouse itself is left and right symmetrical.
Example: you can google and youtube for steelseries Xai or Kinzu (they are ambi mice)
Ergonomic mouse is usually bigger in size, higher hump.
The design is only either can fit for righties or lefties.
Somebody also will think those mice shape is a bit weird compared to other ordinary mice.
Personal opinion, choosing ambi or ergo mouse is based on preference type of grip style.
For palm gripper, resting whole palm on the mouse, move whole arm across the mouse pad will need a ergonomic mouse.
For claw and finger tip gripper need a ambidextrous mouse since the big size of ergo mouse is not allow the user to claw grip it.
Better illustration, better search it from youtube.
I prefer ergonomic mouse. Good for long gaming.