Compared to distilled water, quite a few. However, if you get your water from a main, it is perfectly safe to drink almost all the time. Dangerous contamination is so rare that it makes the news. If you get your water from a well, it is probably just as safe.

Notice that I said "dangerous" contamination. Some drinking water is hard, some soft. Are you going to call the difference "contamination"?

Quite a lot. Tap water contains vitamins, chlorine, some bacteria, dust and fluorine that might be added by your city council. You can distill it to leave behind the foreign substances but the vitamins are left behind. Normal boiled water is best as it leaves in the harmless things and kills bacteria