The amount of pillows that should be on a bed is at least one pillow per person sleeping on the bed.
So for example if you have a full sized bed or mattress and two people sleep on it then you need at least 2 pillows.
Or if it's just you or one person sleeping on the full size bed then you should have 2 pillows so that if you roll over to the other side of the bed you'll still have a pillow to lay your head on.
I sleep on a full size mattress myself because I like the room of it and I have 2 pillows which is enough for me.
Sometimes I wake up on the other side of the bed and sometimes I'm laying on the bed down the middle of it with my legs hanging off the side of the bed.
On a twin mattress I used to just have one pillow.
Some people have 3 or even 4 pillows on their full size mattress which is also okay and depends on what they are comfortable with.