2 mammies, 3 or 4 mediums, and an APC or humvee. The APC is to take out annoying infantry, but can also be used to sneak some engineers in during all of the confusion once your Rush Force arrives at the Nod base.

The 8th vehicles should be a medium tank back at the GDI base to protect against any sneaky Nod units trying to get in.

Tactics vary a lot by map though and the above only applies to maps with the defense towers. In most maps, a force of 4 mammoths is sufficient to severely cripple the Nod base if not destroy it. On maps without defense, though, GDI has a lot of work simply to defend against stealth.