Head of Cattle
Cattle are counted by the head; manufacturing labourers by hands, as “How many hands do you employ?” horses by the nose (See Nose); guests at dinner by the cover, as “Covers for ten,” etc. (See Numbers, Hand.)
In contracting for meals the contractor takes the job at so much “a head” —i.e. for each person.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894