If I'm using an RTC and ATMega328p (arduino UNO chip), do I need two crystals?
No, you do not. Most AVRs, including the ATmega328P, have an internal 8MHz RC oscillator that, while not perfectly accurate, is certainly good enough for simple operation. Simply configure the fuses to use this internal oscillator instead of an external crystal and you're set.

Another option is to use a RTC that can generate a high-frequency square ware and configure the MCU to use this as an external oscillator. It won't run as quickly as with the internal oscillator, but it may be enough depending on the application.

The RTC, on the other hand, will require a crystal. You cannot get around this.