Does the yogurt set from 500 ml milk result in 500 g yogurt?
Making yogurt means letting lactic acid bacteria alter the texture and chemical composition of milk by digesting lactose and producing lactic acid, which in turn interacts with the proteins in the milk, causing the milk to thicken and taste sour.

Unlike in cheese making you are not separating curds and whey, so you are not "losing" significant amounts of substance.

Yes, there might be some fluctuation - an increasing number of lactobacillae and them eating lactose - but on a very small level.   

For our general kitchen precision, 500 g milk makes 500 g yogurt.

 Side note: 500 ml milk is not exactly 500 g, in fact, it's 510 g according to my sources, but I consider these values precise enough for general cooking purposes. Otherwise, you can't even stir, as minute amounts will remain on spooons or in your pots and pans. This is a kitchen, not a lab.