I've had to deal with conversions like this a lot, and 500 milliliters is equivalent to approximately 16.9 fluid ounces. To quickly convert between metric and US customary measurements, I usually just remember that 1 liter is about 33.8 ounces, so I can work from there to figure out other conversions. For smaller amounts like milliliters, I use the fact that 30 milliliters is roughly 1 ounce, which makes it easier to estimate, and then I can refine the estimate if I need a more precise conversion.