To determine how many 1000 ml (or 1 liter) containers can fit into 20 liters, you can use the following calculation:
1 liter = 1000 ml
20 liters = 20,000 ml
Now, divide the total volume in milliliters by the volume of one container:
\[ \frac{20,000 \text{ ml}}{1000 \text{ ml}} = 20 \]
So, there are 20 containers of 1000 ml in 20 liters.