It depends on which Teensy you have.
If you have a Teensy 4.0/4.1 and the revision D audio shield, you use the first column, i.e.:
- CS, pin 10 (alternate pins, any not used by the audio shield)
- D/C, pin 9 (alternate pins, any not used by the audio shield)
- MOSI, pin 11 (required for the first SPI bus)
- MISO, pin 12 (required for the first SPI bus)
- SCK, pin 13 (required for the first SPI bus)
- T_CS, pin 8 (alternate pins, any not used by the audio shield)
- T_DIN, pin 11 (required for the first SPI bus)
- T_DO, pin 12 (required for the first SPI bus)
- T_IRQ, pin 2 (alternate pins, any not used by the audio shield)
If you are using Teensy 3.2, 3.5, or 3.6 without the audio shield you use the pins as above, with the following restrictions:
- CS, pin 10 (alternate pins, 9, 15, 20, or 21)
- D/C, pin 9 (alternate pins, 10, 15, 20, or 21)
If you are using the Teensy 3.2, 3.5, and 3.6 with the revision A-C audio shield, then use the pins from the second column.
- CS, pin 21 (alternate Pins: 15, or 20)
- D/C, pin 20 (alternate pins: 15, or 21)
- MOSI, pin 11 (required for the SPI bus)
- MISO, pin 12 (required for the SPI bus)
- SCK, pin 13 (required for the SPI bus)
- T_CS, pin 8 (alternate pins, any not used by the audio shield)
- T_DIN, pin 11 (required for the SPI bus)
- T_DO, pin 12 (required for the SPI bus)
- T_IRQ, pin 2 (alternate pins, any not used by the audio shield)
In reading the list at pjrc.com, they mention using pins 9 or 10, but these can't be used since they are used by the audio shield. Also, pin 15 may or may not be an issue, since the audio shield has pinouts to hook up a potentiometer to it, and I believe it has extra capacitors and resistors on the line.
The reason is the Teensy 3.2, 3.5, and 3.6 has a set of special pins that can optimize the performance of displays (if the display driver has the optimization). Both CS and D/C must come from those pins. The Teensy 4.0 and LC do not have these optimizations in the hardware. The Teensy 4.0 has only one special pin (10), but to do the optimization you need to use two special pins. The Teensy 4.1 has 3 special pins (10, 36, and 37), but I'm not aware that the Teensy 4.x driver has the support that the Teensy 3.x driver did for the special pins.
Now to get to the nitty gritty, there are a few more special pins on the Teensy 3.2, 3.5, and 3.6. But these pins have restrictions (if you use one of these other pins, you can't use specific pins). But if you are curious, go here (in particular reply #5):
Now, if you are using Audio shield revision A-C on the Teensy 3.2, 3.5, or 3.6, then you need to use alternate pins for MOSI (7) and SCK (14). The reason is the I2S pins used by the audio shield overlap with 2 of the 3 the SPI pins, and you have to have use the alternate SPI pins. The Teensy 4.0 and 4.1 do not have alternate SPI pins for the first SPI bus.
Because of the difference in the I2S pins and the alternate SPI pins, you must use audio shield revision A-C for the Teensy 3.2, 3.5, and 3.6. For the Teensy 4.0 and 4.1, you must use audio shield revision D.
I believe the Teensy LC does support the same alternate SPI pins that the 3.2, 3.5, and 3.6 do, but since you can't use most of the audio shield functions on the LC, it is likely you wouldn't be using that board.