There are a couple of options you can choose. To spice it up you might try spiced rum or brandy. Add a shot or two of hot damn, fire water, or aftershock to add acinnamon flavor. Iuf you dont want it to taste stong like liquor try baileys, Kahlua, frangelico, or drambuie. there is a compnay that makes cherry and vanilla flavored whiskeys. If you think pepperint would be nice try pumpleminz. a cinnamon stick or an orange cut in half would be a nice extra.

Traditionally it’s brandy.
The amount varies with how strong you want it and how much you want it to effect the flavor.
I would add around 1 cup brandy per quart eggnog. But test it first.

4 cups of whiskey, preferably crown royal