The size and number of horses you will need to work your organic farm will depend on the size of your operation, the nature of the work, and your personal preferences. If all you have is a 2-acre CSA garden, then a team of ponies may well do all your work and eat a lot less feed, but if you have a lot of heavy draft work to do or many acres to cover you may need the biggest draft horses you can find. Generally a draft horse can exert 10% of its body weight in a horizontal pull (the definition of draft) on a steady basis. Thus a 1,000-pound horse could exert a 100-pound pull all day, but a 2,000-pound horse could pull twice that.
The same horse can exert a pull of half its own weight for a brief time to get you out of a tight spot. As an example,
a plow cutting 12" wide, under average soil conditions, would require a draft of 378 pounds. Thus three 1,000-pound
horses or two 2,000-pound horses could be expected to pull this plow. If the soil is clay or dry, or the crop is alfalfa,
the draft could be much higher. If the horses are not well conditioned to work they will not be able to work without
frequent rests and they run the risk of getting sore shoulders.
The general rule of thumb is 25 acres per horse, so a 50-acre mixed farm could be operated with a team of two, but having a spare horse is wise to lighten the load or take the place of a sick or sore horse, or a foaling mare.
Any breed can be suitable, depending on your preferences and prejudices. I personally prefer short but heavy draft horses. Short because they are easier to harness on a daily basis, and heavy because we have a lot of heavy field work to do.