Hi Pump,
Welcome to this forum. Differential op amp can be build on various way by using 1 to 3 op amp. The CMRR is frequeny depending.
If your desgin is theoretical you can be in a perfect world an choice the 1 opamp design with very tight components specifications. Practical design would probably be a 2 or 3 op amp one.
Just google a little bit and you will find many informations, mainly on analog devices and Texas Instruments sites. TI even offer a free simulation package.
Olivier