Last time I hallway tested it, people couldn’t tell what prime numbers are, and to my surprise even the ones with tech/math-y background forgot it. My results were something 1.5/10 (ages 30+-5) and I didn’t go to cabinets where I knew there are zero chances.
But there's a difference between "knowing what the formal definition is" and "having a feeling that a number is somehow unique due to it's indivisibility".