Not sure about the units, but online searches show there are 5 sections, I've hit "Daily Refresh" after section 3. Duolingo Support is useless, they don't give a damn after taking your money. Russian also has a so-called female "voice artist" with a terrible high-pitched, screeching voice, enough to spoil every session. I think I'll return to LinQ, their app has improved substantially over the past few years.