In the first example we would say Louis the sixteenths reign. In the Second John the tenths reign.

These are Roman numerals representing ordinal numbers (first, second, third, etc). Louis XVI is pronounced as “Louis the sixteenth.” John XX is pronounced as “John the twentieth.” In Roman numerals, I = 1, V = 5, and X = 10. https://en.m.wikipedia.org/wiki/Roman_numerals

Hi, there How do you pronounce the “XVI’s” in Louis XVI’s reign”? What about the XX’s in “John XX’s”? Thank you! Howdy! Loo-ee the sixteenth John the twentieth

Oops.... my apologies! I missed the second X for John 🙄