I have a 14'×30' cabin. How many should be used and what size breaker is necessary?

There are a few factors that go into estimating BTUs needed. Assuming you are well insulated, and have 12' ceilings, and would like to be able to have a 70-degree rise in temp on a 0-degree day; you will need a minimum of 45,000 BTU output. You can go up from there to easy equipment work load and if any factors involved are not optimal.