Hundreds of people are lining up to support Patrick Mahomes' fiancee ... 'cause TMZ Sports has learned thousands of "Team Brittany" shirts have already sold in just one day.
A rep for Charlie Hustle -- the shirt company that partnered with Brittany Matthews to make the tees in the wake of her Arrowhead Stadium champagne celebration backlash -- tells us the shirts have sold so quickly ... there's now a waiting list for the item.
In total, through the first 24 hours-or-so of sales, the rep said more than 2,000 units have been bought.