There isn't a definitive number of colleges in the US that *exclusively* offer game design degrees. Many universities offer game design programs within larger departments (like computer science, digital media, or fine arts). Some colleges might have a game design focus within a broader program, while others offer it as a specialization or concentration.
Therefore, trying to count them precisely is impossible without a very specific and potentially subjective definition of "game design college." You'd find hundreds of colleges offering relevant courses and programs, but far fewer that exclusively focus on game design as their primary offering.