The first two that I did I used 1/4" baltic birch. Then I tried stacking two pieces of 1/8" baltic birch. I prefer the look of 1/8" thick portraits. Matter of personal preference. I prefer to stack only two pieces but others do more. Maybe as I get better, I will feel more comfortable with more stacked.
I don't find portrait cutting hard. It can get tedious and time comsumming, but I really enjoy it. The two pieces of advce that I received that really helped me get better at it were: keep your shoulders down in a relaxed position. If your shoulders are up, you are tense. Second, concentrate on breathing at a normal rate. I was going to say breath normal, but I wear a dust device that I have to brearh through my mouth, which isn't normal for me.
A third thing I was encouraged to do when I wanted to start trying more intricate portraits/scenes was only worry about one hole at a time, don't be intimidated by the number of cuts just go one at a time.
One more thing I just thought of. When drilling holes to stick the blade through, I start with a 1/16" bit and drill all the holes I can with that large of a bit. I only drill the smaller holes, recommented for the size of blade I use where a larger bit would be too big to drill in the area to be cut out. I hate trying to thread the blades through those tiny holes!
Get a varity of good blades to try out. You will find one type you prefer and works best for you. Include some spiral blades to try. They are not for everyone, but I love them.
Welcome to the forum and have a good time.