You absolutely cannot visualize the concept "2" in your head. You can only visualize "two things" or the word "two" or the symbol "2", but the actual concept "2" is just as abstract as a real number.

You might say, I can imagine 2 apples, but I can't imagine pi apples, but you could just as easily imagine unrolling a circle with a diameter of 1, and you have visualized "pi" just as well as you can visualize 2 apples.