Here are the six major varying ethnicities, by that I mean heavily varying facial features between the six. Your first 50 or so drawings should consist of these, spread out between different age groups (aim for kids, adults, elderly)
For the next 30, pick certain variations of the major groups and try more drawings of them. Eg: Central Asians vs. Southeastern Asians...whatever you want. (My god its hard not to somehow sound offensive in this entry..)
These 80 drawings can be anywhere from line art to rendered shit like the one I did above, up to you.
And....for the last 20, go friggin crazy! Cartoonize (try not to get offensive here) in any styles and mediums you like. And here's the hard part. Do it from memory, not references.
Ready, set go!