I guess I’m sort of uncommon here then! I always sleep on my back. It’s the position I feel most like actually properly falling asleep. Even if I am extra tired and fall asleep on my side, when I wake up I’ll have moved onto my back during my sleep.
A combination of side and back. Originally I was side only but I try to sleep on my back as much as I can as it offers many benefits. Plus I have posture issues and sleeping on my back (with no pillow) helps to straighten the spine and neck). It definitely took some getting used to though and I still switch to my side from time to time.