I don't think the number of trees really matters. A fixed amount of energy from the sun falls onto a certain area and plant life will emerge to cover all of that area and capture the energy. If you plant fewer trees, each will get a bigger share of the energy and end up larger. If you plant more trees they will each get a smaller share of the energy and end up smaller. A closed canopy is a closed canopy and more trees will not result in more energy/photosynthesis/firewood.
As far as what area is needed to cover your needs:
"Primary productivity" is the term used to describe the rate at which energy is converted into organic material via photosynthesis.
Deciduous temperate forests have a net (i.e. useful) primary productivity of:
- ~6000 kCal/m2/year
- ~16 kCal/m2/day
- ~67 kJ/m2/day
- ~0.0186 kWh/m2/day
The households in my country average about 24kWh/day of energy usage over the course of a year. In the temperate regions of the country, about 30% of that (7.2kWh/day) is spent on heating. To provide that energy would require:
7.2/0.0186 = 387m2 of deciduous temperate forest
Thus the yearly heating needs of a single household can be provided by 'logging' an area only about 20x20m in size — roughly double the size of the house itself.
Of course you wouldn't be logging the same 20x20m area each year, you'd be logging a different 20x20m area each year — that gives time for trees to (re-)grow in previously-logged areas.
Tweak the numbers above to suit your local situation and plant (or allow existing trees to sucker) at least that area each and every year and you'll be right.
If your trees reach harvestable age in (say) 20 years, then you're logging on a 20-year rotation and would need a woodlot 20x the annual logged area (387m2) for it to be sustainable (i.e. a woodlot at least (20x387=) 7740m2 (0.774Ha) in size).
As others have suggested, it's a good idea to mix up the species. Not only do you end up with different burn characteristics, but if your woodlot is hit by a disease or invaded by insects, there's less chance that all of your trees will be wiped out at the same time.