There were 8 carriers at the start of WW2. They were the Langley, the Lexington, the Saratoga, the Ranger, the Yorktown, the Enterprise, the Wasp, and the Hornet. The Ranger was the first whose hull was laid down as a carrier. The ones before were converted from other types of ships.
The reason that the United States had only eight carrier at the start of the war is that carriers are very expensive and very powerful and there was no apparent need for more in peacetime. Some people would have preferred more but some would have preferred less (some senior naval officers still doubted the usefulness of carriers) so we had eight. Japan only had about twelve if I remember correctly. We began a crash program to produce more as soon as the war began. We produced an incredible number of them. When the war ended we had ninety nine in commission. Since we lost some during the war we must have built close to a hundred of them. Michael Montagne