Only five of the seventy-two are ranked Earl in this collection: Furfur, Halphas, Raum, Bifrons and Andromalius, commanding 124 legions between them. That is the smallest rank apart from the solitary Knight, and it contains the smallest command in the whole catalogue, the six legions given to Bifrons.
The small number is an artefact of how the rank is used, and the artefact is worth stating plainly because it affects any count anyone makes from this data. In the Ars Goetia, Earl is more often a spirit's second title than its first. Vine is a King and an Earl. Murmur is a Duke and an Earl. Ronove is a Marquis and a Great Earl. Botis and Glasya-Labolas are each a President and an Earl. Marax is given as an Earl and a President, and Ipos as an Earl and a Prince.
This collection assigns one rank to each spirit, which is what a single column can hold, and files compound titles under the first of the pair. The five Earls listed here are therefore the spirits for which Earl stands alone. Counted the other way, by every spirit to whom the text gives the title at any position, the rank would be roughly twice the size.
Why Earl in particular should behave this way is not obvious from the text. It may be no more than that comes and its vernacular equivalents were the most elastic of the borrowed titles, general enough to be added to a spirit already described as something else without contradicting it.