It is variable depth - the main ranks also have ranks between them, with some denoted by super- and sub- prefixes to the major categories, and others of unnamed rank defined only by its position between two adjacent levels.

The NCBI's taxonomy browser is good for exploring the various lineages, e.g. here is the data for humans: http://www.ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi?mode...