When you insert a thematic map legend into a map layout, you should have the option to include automatically additional column(s) that display select summary statistics for each map class such as: number of elements (geographic areas), proportion of elements, sum of elements, number of excluded elements.
I always have to get number of elements from under Symbiology -> Classify and then click on each class and look at the number displayed at the bottom of the screen. Then write the numbers down and calculate the proportions/percentages in another program like Excel. Then type them into my map layout with the text tool. Not very convenient.