The answer to this question follows the Binomial distribution where n is the number of people who own the shirt, and p is the probability that one person would wear their XOXO shirt, taken to be one over the average number of shirts (or equivalent: dresses, sweaters, etc.) per person.
This assumes that anyone who owns an XO shirt only has one of them, and also that every day we all pick a shirt (or equivalent) at random.
Furthermore, this calculation does not take laundry into account. Taking laundry into account would be...a mess.
The average number of people currently wearing their XOXO shirt is ??? (put in some numbers)
(It's actually just the first number divided by the second one)