It would depend on farm size. Any farm should have a tractor for day to day operations of the farm. But size matters when choosing the correct type of tractor.
A small farm should only need one tractor as long as it is in good condition.
If not the tractor itself, then it's almost certain the business would have to have some sort of general liability insurance.
A sub compact utility tractor is probably best for your small farm. You can find it here on the John Deere website: http://www.deere.com/en_US/ProductCatalog/FR/category/FR_TRACTORS.html
Depends on your country laws. I.e In the UK a farm vehicle that has to travel on any road other than that of the land the farm is on will require taxation and insurance to do so.
The best tractor for your needs will depend on what all you are going to be doing with it. A tractor is expensive. A site like Mother Earth News has great articles on self sustainable farming and equipment. Here is a link for one of their articles on small tractors: http://www.motherearthnews.com/Sustainable-Farming/1981-03-01/Small-Farm-Tractors.aspx
The gas mileage of a farm tractor depends on the model, the year, the condition, and the land driven on. There is no set gas mileage to be expected from a farm tractor.
A John Deere 4720 tractor is a good choice for a compact tractor for a small farm. To see features of this tractor visit http://www.deere.com/en_US/ProductCatalog/GC/servlet/com.deere.u90785.cce.productcatalog.view.servlets.ProdCatProduct?pNbr=686DLV&tM=GC.
There are many different types of farm tractors out there. You can look in the manual that came with the farm tractor to see how it works and how to maintain it.
They just call it a tractor.
U Can Use Tractor tires to run A farm Tractor
hand tractor is used farm