+1 on above
the biggest problem when planting x-mas trees under hardwood canopy is there is no light or not enough light for the X-mass tree to thrive on , so as they grow they create a thin whispy tree as they compete to reach the light or just eventually die off , either way they don't make an X-mas tree worth selling
the other issue is type of ground and type of tree you intend to plant,
if you intend to actually turn them into a cash crop, it's a lot harder than just planting them in the ground
Pines grow in quite a variety of ground and need least work doing to them but they do need plenty of light to grow properly and are not the most liked as an X-mas tree in this country
Norway spruce also can be grown in a variety of ground but need pruning twice a year in reality and most susceptible to bugs such as green aphid and red spider mite
Nordman fir and Frazer Fir are very popular for x-mas trees due to shape and needle retention but need a lot more maintaining especialy in the bug department
They will all need work done on them atleast once a year from around 3 years old after being planted depending on how well they grow, if you want to produce a saleable crop in 4-5 years time
Douglas fir are a bit of a nightmare tree as they need a lot of work on them to get a decent shape and quite often by working on the leader it fecks them up
Personally if you think you hav a canopy/light issue then don't plant X-mas trees thinking they will be sale-able in a few years time , it won't happen as a rule
as for amount of trees, plant them on a 4ft spacing so they hav room to grow evenly and you hav room to spray and work on them properly
rabbits will be a concern from the start and so will aphids and weeds such as thistles and nettles and to be honest if the deer wanted to get into your pen they will anyway
why don't you pop down to your local nursery/garden centre and see what winter plants they hav going that they are flogging off cheap, I mean the ones they are going to dump anyway
you'll be surprised at how many of those survive once planted and for the cost it won't matter if 50% die