I have a large Magnolia tree that needs pruning. The limbs are almost touching the house and they need to be trimmed back. My question is if I trim off the ends of these limbs will the whole limb die?

Generally, you can prune broadleaf evergreens at anytime of the year. Yet, I would recommend that you prune in early Spring. Pruning in the fall can sometimes encourage new growth that won't be hardened off before cold temperatures arrive and the new wood could die. It is very unlikely that the whole limb would be winter-killed in any case.