There have been four kings named William in British History.
William I (the Conqueror) reigned as King of England from 25th December 1066 until his death on 9th September 1087.
William II, son of William I, reigned as King of England from 9th September 1087 until his death on 2nd August 1100.
William III, "William of Orange" reigned as King of England and King of Ireland from 13th February 1689 jointly with his wife Queen Mary II until her death on 28th December 1694 and then reigned alone until his death on 8th March 1702. He was also William II of Scots from 11th April 1689 until his death.
William IV reigned as King of the United Kingdom of Great Britain and Ireland from 26th June 1830 until his death on 20th June 1837.