See on file NMEA2000_CAN.h:
There are also library specific defines:
mcp_can:
#define N2k_SPI_CS_PIN 53 // Pin for SPI Can Select
#define N2k_CAN_INT_PIN 21 // Use interrupt and it is connected to pin 21
#define USE_MCP_CAN_CLOCK_SET 8 // possible values 8 for 8Mhz and 16 for 16 Mhz clock