Choosing crystal, strangely narrow requirements in MCU datasheet?
You are misreading what it says in the data sheet I believe: -



The MCU spec is 17 pF maximum.

The XTAL they chose to demonstrate how to calculate \$C_{LEXT}\$ has a requirement for a load of between 12.5 pF and 17 pF. This isn't an MCU spec as such.

So, choose a 12pF crystal and calculate \$C_{LEXT}\$ based on the formulas.