The prefixes are given from the least significant decimal digit up: units, then tens, then hundreds, then thousands.
IUPAC prefers the spelling icosa- for the affix corresponding to the number twenty on the grounds of etymology.
However both the Chemical Abstracts Service and the Beilstein database use the alternative spelling eicosa-.
There are two more types of numerical prefixes in IUPAC organic chemistry nomenclature.
[1] Numerical prefixes for multiplication of compound or complex (as in complicated) features are created by adding kis to the basic numerical prefix, with the exception of numbers 2 and 3, which are bis- and tris-, respectively.