。
There were six sheep in the field.
Literal
Field [in-には] 6-[counter-頭] [gen-の] sheep [subj-が] existed-[いた].
The counter 頭 is used for large animals — cattle, sheep, horses, elephants, tigers, dogs (large ones). Small animals use 匹. Here 六頭 ('six head') acts adjectivally before 羊. The existence verb いる is used for animate subjects.