Most people don't pass out on mushrooms, although it is possible. The biggest concern with this drug is that it is a hallucinogenic and taking too much of it could put you at risk. This risk comes into play by overreacting to what feels like legitimate fears but to the outsider are totally irrational.
If you do pass out while taking mushrooms, it's likely you took too much but you'll come down and be fine after a while.
It is possible for you to pass out while taking mushrooms, especially if you do a lot of them. Typically people do not pass out while on shrooms if they take a normal amount though. Like any drug, you need to be careful not to take too much. You also need to be careful about where you are taking shrooms.
Make sure you are with people you trust and that you have a safe way to get home. This is especially important if you have never done them before and if you plan to consume enough that you might pass out.