19 maggots per 100 grams of drained mushrooms is ok! If the FDA finds 20 or more maggots per 100 grams of drained mushrooms they will recommend legal action.However, only 4 maggots are allowed if they are larger than 2 millimeters.
For complete details from the U.S. Government: Mushrooms and Maggots
I realize you are probably completely disgusted to learn that your delicious mushrooms might have 19 small maggots or 4 of the big ones... but some might not even consider those quantities to be bad... next time consider ordering a pizza with mushrooms and maggots!
Maggots may even be able to improve human nutrition. A number of studies have shown that maggots can be valuable but inexpensive, protein-rich supplement for chickens in impoverished regions. see: Insects to lean about insects for food.