Read Datasheet from the Crystal Manufacturer, it will specify Load Capacitance CL value,
that was used when when the crystal was trimmed during production.
Manufacturers may deliver very similar crystals trimmed to be used with customer specified values of load capacitance.
What may make it confusing, is that CL value specified is the effective capacitance of load,
as seen by the crystal.
When a Pierce oscillator circuit is used, as in most microcontrollers, the 2 capacitors are acting as connected in series, so you must calculate the resulting capacitance of the 2 capacitors, then add capacitance of wiring, PCB traces, pins and bonding wires inside the IC package.
A good oscillator amplifier circuit have some kind of gain control (AGC).
It usually will tolerate a wide range of load capacitance, the main effect beeing that different load capacitance will pull resonance frequency a little away from trimmed nominal frequency.
Thus, there are some circuits that may use 2 voltage controlled capacitance diodes to trim oscillator frequency.
Then microcontroller designers and manufacturers, compete in making crystal oscillator amplifier circuits that may run with low power consumption, so much that Secondary Crystal oscillator circuits in PIC32MZ and PIC32MK devices are not expected to work at all. See errata documents for those devices.
20 nanoseconds accuraccy as mentioned in message #12 is meaningless.
20 nanoseconds in what time interval?
20 nanoseconds in 1 second, cannot be achieved with a crystal oscillator, without a lot of trickery,
or probably not at all. https://www.microchip.com/design-centers/clock-and-timing/atomic-clocks