Altogether the Japanese built 10 liners that were designed to be converted to aircraft carriers in an emergency.
According to Guy Robbins in The Aircraft Carrier Story 1908-45, the 3 Asama Maru class liners of 16,675 G.R.T. and 20.7 knots were subsidised by the Ministry of Transport. They were commissioned 1929-30. The Navy wanted to convert them to auxiliary aircraft carriers (XCV), but they were sunk before the shipyard capacity to do it became available. The 3 ships were sunk in the following order: Tatsuta Maru, 09/02/1943; Chichibu Maru 28/04/1943; and Asama Maru 01/11/1944.
Once more according to Robbins, when the USA began the construction of large merchant ships, Japan responded with a law to subsidise first class merchant ships. In its first phase twelve liners were built five of which were designed for conversion to aircraft carriers. These were the Nitta Maru and Kashiwara Maru classes.
3 Nitta Maru class of 17,830 G.R.T. and 22.2 knots were subsidised for the Japanese Mail Steamship Company's (NYK) European service in 1931 (but another sources I checked that against say that NYK decided to build them in 1937, which is more plausible as Nitta Maru and Yawata Maru were laid down in 1938). Kasuga Maru was laid down on 06/01/1940, launched on 19/09/1940, taken over by the IJN in May 1941 when she was still fitting out and completed on 15/09/41 as the XCV Taiyo. Nitta Maru (completed as a liner on 23/03/1940) and Yawata Maru (completed as a liner on 31/07/1940) was requisitioned by the Navy in February 1941 and in October 1941 respectively. Yawatta Maru was immediately converted into the XCV Unyo (25/11/1941 to 31/05/1942) and Nitta Maru served as a troopship before being converted into the XCV Chuyo (20/08/1942 to 25/11/1942). Chuyo the last conversion had the shortest career as an aircraft carrier because she was sunk on 04/12/1943, Unyo was sunk on 16/09/1944 and Taiyo on 18/08/1944.
2 Argentina Maru of 12,755 G.R.T. and 21.5 knots were built of the Osaka Ship Company's (OSK) South America service in 1932 (but Wikipaedia says they were ordered in 1938, which is more plausible because the Combined Fleet website says both ships were completed in 1939 and Argentina Maru was launched on 31/05/1939). Both were requisitioned by the IJN in May 1941 and used as troop ships, for example they were both employed as transports during the Battle of Midway. Argentina Maru was converted to an XCV between December 1942 and 23/11/1943. She survived the war and was broken up in 1948. Her sister ship Brazil Maru was sunk on 05/08/1942 before she could be converted, according to the Combined Fleet website she was on her way to Yokosuka for her conversion.
The bonus ship was the German liner Scharnhorst of 18,184 G.R.T. and 22.0 knots, which was converted into the XCV Shinyo. She was purchased in February 1942 and her conversion was completed on 15/12/1943. Her career as an aircraft carrier lasted less than a year because she was sunk on 14/11/1944.
According to Wikipaedia the 2 Kashiwara Maru class liners of 27,500 tons and 25.5 knots were ordered in late 1938. They were laid down in 1939. They were officially purchased in February 1941, but unofficially in October 1940. Both ships were launched in June 1941 and completed in the middle of 1942. Although they carried double the number of aircraft as the other XCV and were fast enough to operate with the fleet their hangars were only 16 feet high. On the other hand the Ark Royal and IIRC the first 4 armoured carriers had hangars that were 16 feet high and the hangars on the Implacable class were only 14 feet high. Hiyo was sunk on 20/06/1944 during the Battle of the Philippine Sea and Junyo was damaged. According to Wikipaedia, after repairs at Kure, Junyo remained in the Inland Sea without aircraft until 27 October when she was tasked to transport material to Borneo. (The Battle of Leyte Gulf was from 23rd to 26th October so she probably didn't take part because she was still under repair and/or lack of aircraft). She survived two unsuccessful attacks by US submarines before being torpedoed on 09/12/1944 and repairs were abandoned in May 1945 for lack of materials, but survived the war and was scrapped in 1946.
What I find interesting about the above...
is that the 7 ships of the Nitta Maru, Argentina Maru and Kashiwara Maru classes were all ordered after 31st December 1936. That is after the aircraft carrier tonnage quotas imposed by the Washington Treaty had been abolished. Therefore the Japanese could have built 7 proper aircraft carriers instead of them (say 2 Shokakus and 5 Unryu class) but it's likely that the Americans would have built more aircraft carriers in retaliation. They might have brought the whole Two Ocean Navy programme forward 2 years.
Based on what has been discussed so far I now think that the IJN should have given an even bigger subsidy to the shipping lines that ordered the Nitta Maru and Argentina Maru classes to order faster ships that could work with the fleet like the Kashiwara Maru class. Preferably they would have been additional ships of that class, but the slipways probably weren't big enough. Furthermore the IJN should also have converted them to aircraft carriers sooner so that as many as possible would be in service by the end of 1941 to deliver the heaviest possible knock out blow.
Therefore Argentina Maru, Brazil Maru and Nitta Maru should have been converted to an XCV immediately after they were requisitioned in the first half of 1941. In the case of Nitta Maru that might bring her commissioning as the Chuyo forward from the end of November 1942 to the end of May 1941. Although Argentina Maru's conversion IOTL took nearly a year I think she and Brazil Maru could have been converted in 3-4 months like Nitta Maru and Yawata Maru IOTL which would mean they would be ready by the end of 1941 too. The earlier conversions of Nitta Maru and Yawatta Maru would have allowed 2 more auxiliaries or 2 of the older fast liners to be converted in 1942.
So instead of 10 aircraft carriers at the end of 1941 (6 fleet carriers, Ryujo, Zuiho, Hosho and one slow XCV) they would have 14, that is the 6 fleet carriers, Ryujo, Zuiho and Hosho - plus 5 XCV type aircraft carriers operating 135 aircraft, which, were fast enough to operate with the fleet for the battles of 1942.