You will need a total of 2 - if you already have one on your bot, that will suffice for one of them. The other will be needed to convert commands from the PC into RC signals.
The RC "PWM" protocol is usually implemented as a pulse at 50Hz, with a pulse width between 1000ms and 2000ms. The linear scale in between corresponds to the linear movement of the joystick on the RC transmitter.
To send the signals, you will probably find it easiest to use the "trainer" connection on your transmitter and figure out the necessary protocol to emulate another controller (this part is transmitter-dependent, so I can't help much).
To receive, you simply hook the signal pins from the receiver into GPIO, and time the pulse. You can then use the value as an input.
Here's a link that might help: http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1193927390