廿
廿 – Nian4 – Twenty
For completeness I’ll also mention that there is a single character for twenty: 廿. I think, but am not entirely positive, that this character and 卅 – Sa4 – Thirty are mainly used as shorthand for dates, especially on calendars.
For more on the horrors of the Chinese calendar, see here. One nice thing that I’ve never run into before is the names of the moons. The Chinese didn’t stop with naming all the full moons, they even named all the new moons. Right now is “Vernal Equinox” which isn’t that interesting but we just came out of “Excited Insects”, and are heading into “Clear and Bright”. Sounds nice. These names should really be added to the authoritative Names of the Moons site.