Do I need a crystal to for the Atmel SAM D20 MCU to Run at 48 MHz?
From the SAMD20 datasheet, you have the internal 8MHz RC clock. 

This can be routed to the generic clock multiplexer 0, which is used as source for the DFLL48M.

Please note that the RC oscillator could be imprecise (EDIT: precision is greatly improved by loading factory calibration parameters), so it will be the DFLL, but you stated that you can live with this.

To increase accuracy, you can use the 32768 Hz oscillator (with a 32768 Hz external crystal) and the DFLL. 

On SAMD21 (i.e. not your case), you can also use the USB start of frame (1ms) as synchronization (in device mode), without having to use any crystal!