1806 Half Cents are found with two different obverses: Small 6 and Large 6. On the Small 6, the 6 of the date is completely separated from the bust; on the Large 6, the top of the 6 is embedded in the bust. The Large 6 obverse is combined only with a With Stems reverse, resulting in the Cohen 4 variety. This, along with the Cohen 1 (Small 6, Stemless) are the two most common varieties of the year.
The 1806 Large 6, Stems Half Cent is relatively common in Mint State, including Brown and Red-Brown examples. Full Red examples are known, but are extremely rare.
Sources and/or recommended reading:
"American Half Cents (Second Edition)" by Roger S. Cohen, Jr.