It's hard to give a number, because the rapid tests answer the question, “Am I contagious?” and not “Am I infected?” They answer the question, “Am I contagious right now with COVID-19?”
They're actually very good at addressing that question. You might be a positive case but not contagious to others because you haven't built up enough virus in your system, and your rapid test will be negative. Again, the test answers the question, “Am I transmissible to other people?” It's very good at doing that.