A farm has a natural balance.  Fields grow crops.  Mills turn the crops into livestock feed.  Livestock produce dung and products that can be sold at market.  Dung is turned into fertilizer.  Orchards use fertilizer to produce leaves, nectar (later in the game) and products to be sold at market.  Nectar is turned into honey for building materials and market.  Orchard products generate over 70% of a small farms income.  Yet, an orchard generates $0 when you have no fertilizer.  The general rule I suggest is 2 animals for 1 orchard.  The more time you invest into your farm, the lower the ratio can be and the less you farm, the more animals you need.  

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Working with a friend that mentors and coaches many starting farmers, we are realizing that you need 3 or 4 stables to every orchard in the begining.