{enc28j60} Is the 50ppm crystal good enough
Information that critical to the usage of the chip is is always in the datasheet. If it can be summed up in a couple of numbers and you can't find it in the main table of specifications (if there is one at all), then it's usually under the "Electrical Characteristics" section near the end of the datasheet. That is exactly where it is in this case, under that section on page 81, Table 16-2.