In the second half of 1941, the Australian I Corps was concentrated in Syria and Lebanon to rebuild its strength and prepare for further operations in the Middle East. Following the outbreak of war in the Pacific most elements of the Corps, including the 6th and 7th Divisions, returned to Australia in early 1942 to counter the perceived Japanese threat to Australia. The Australian Government agreed to British and United States requests to temporarily retain the 9th Division in the Middle East in exchange for the deployment of additional US troops to Australia and Britain's support for a proposal to expand the RAAF to 73 squadrons. The Government did not intend that the 9th Division would play a major role in active fighting, and it was not sent any further reinforcements. All of the RAN's ships in the Mediterranean were also withdrawn to the Pacific but most RAAF units in the Middle East remained in the theatre.In June 1942, four Australian N-class destroyers were transferred to the Mediterranean from the Indian Ocean to participate in Operation Vigorous, which was an attempt to supply the besieged island of Malta from Egypt. This operation ended in failure, and Nestor had to be scuttled on 16 June after being bombed the previous day. After this operation, the three surviving destroyers returned to the Indian Ocean.In mid-1942, the Axis forces defeated the Commonwealth force in Libya and advanced into north-west Egypt. In June, the British Eighth Army made a stand just over 100 km west of Alexandria, at the railway siding of El Alamein and the 9th Division was brought forward to reinforce this position. The lead elements of the Division arrived at El Alamein on 6 July and the Division was assigned the most northerly section of the Commonwealth defensive line. The 9th Division played a significant role in the First Battle of El Alamein which halted the Axis advance, though at the cost of heavy casualties, including the entire 2/28th Infantry Battalion which was forced to surrender on 27 July. Following this battle the division remained at the northern end of the El Alamein line and launched diversionary attacks during the Battle of Alam el Halfa in early September.In October 1942, the 9th Division and the RAAF squadrons in the area took part in the Second Battle of El Alamein. After a lengthy period of preparation, the Eighth Army launched its major offensive on 23 October. The 9th Division was involved in some of the heaviest fighting of the battle, and its advance in the coast area succeeded in drawing away enough German forces for the heavily reinforced 2nd New Zealand Division to decisively break through the Axis lines on the night of 1/2 November. The 9th Division suffered a high number of casualties during this battle and did not take part in the pursuit of the retreating Axis forces. During the battle the Australian Government requested that the division be returned to Australia as it was not possible to provide enough reinforcements to sustain it, and this was agreed to by the British and US governments in late November. The 9th Division left Egypt for Australia in January 1943, ending the AIF's involvement in the war in North Africa.

Question: how many destroyers remained and returned to the Indian Ocean?
Answer: three