This is from Wikipedia - but it coincides with my recollection. It was the occupation forces that were in dispute, although the argument game during the planning of the invasion (which, of course, is when it should occur).
“Shinseki publicly clashed with Secretary of Defense Donald Rumsfeld during the planning of the war in Iraq over how many troops the United States would need to keep in Iraq for the postwar occupation of that country. As Army Chief of Staff, General Shinseki testified to the U.S. Senate Armed Services Committee that “something in the order of several hundred thousand soldiers” would probably be required for postwar Iraq. This was an estimate far higher than the figure being proposed by Secretary Rumsfeld in his invasion plan, and it was rejected in strong language by both Rumsfeld and his Deputy Secretary of Defense, Paul Wolfowitz, who was another chief planner of the invasion and occupation.[15] From then on, Shinseki’s influence on the Joint Chiefs of Staff reportedly waned.[16] Critics of the Bush Administration alleged that Shinseki was forced into early retirement as Army Chief of Staff because of his comments on troop levels; however, his retirement was announced nearly a year before those comments.[17]
“When the insurgency took hold in postwar Iraq, Shinseki’s comments and their public rejection by the civilian leadership were often cited by those who felt the Bush administration deployed too few troops to Iraq.[18] On November 15, 2006, in testimony before Congress, CENTCOM Commander Gen. John Abizaid said that General Shinseki had been correct that more troops were needed.[18]”
That’s my recollection too about the required number of troops.
I think a problem was that Rumsfeld and Wolfowitz were not taking into account postwar insurgency. They were focused on Saddam’s removal and his Baathists only, and probably thought they could then move to ‘nation-building’ phase. Whereas the U.S. needed to dominate Iraq completely, before ‘nation-building’.