Hi,
while 20pF is a typical value for a 16MHz crystal and will almost definitely work ok for most crystals,
the correct value of the capacitor to use can be found in the datasheet for the crystal you use.
You need to know the loads from the circuit it's driving as well, as it's the total loads described in the X-tal sheet and you need to deduct the intricate part of the total.
The best way, especially if you don't wanna have to "pair" the datasheets, is to experiment, as the X-tal will be most stable with the least capacitive load, but it need some capacity to start.
When the lowest value giving stable start is found, add around 20% to that value to counter for changes in conditions (temperature, humidity, Murphy etc.).
For the best precision, use a variable cap for one of them and dial in the precise frequency (cannot be done without a very precise frequency counter or eg. a radio based reference oscillator). An X-tal oven is very helpfull for stable precision as well.
