If the number line is like a single axis, like an x axis or a y axis, then the point 0 on the number line separates the positive numbers from the negative numbers. Let's say the positive numbers are on the right of zero and the negative numbers are on the left. When a number is reflected it changes sides. A positive number goes from right to left and a negative from left to right. So reflection is simply negation: x goes to -x. For example, when x=2 its reflection is -2; when x=-10 its reflection is -(-10)=10.