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You need two forms of grounding according to the code. Grounding to the water main is typically the best ground in a building. If you don't have a water meter you can add an extra ground rod.
Unless otherwise specified you need a 5/8th inch by 8 foot copper clad rod. A great trick is to drive it in at a 45 degree angle and bend it up as you go so it ends up straight after it drive in all the way. This keeps you from hitting hard pan and keeps you from having to get on a ladder to drive the rid which can be kinda dangerous. Leave about 4 inches out of the ground for you ground clamp. For a 200 amp service use #4 armored ground. For 125 amp or 100 amp you can use #6 armored ground wire and an armored ground clamp. The NEC allows that for ANY connection to ground rod, #6 solid copper is the largest conductor required. The ground rod is SUPPLEMENTAL, and only in rare circumstances is the ground rod your primary ground. The sizes given above are for your primary grounding connection, such as a metal water line.
You will need two 8 foot ground rods. One placed under the meter base and another one placed 6 feet to the side next to the foundation. If this is a home I highly recommend you install a 200 amp service instead of a 100 amp service. Not much more expensive and you will have plenty of room for growth.
As long as the clamp is of an approved type only one is needed per ground rod.
The NEC makes no provision for using 1 ground rod even on a temporary service. According to the NEC you must use 2 ground rods. However in my area we are allowed by the local inspector to use just 1. Call your local inspector and see what he requires as he has the last word on this.