The Prophet ﷺ used to observe I'tikaf during ten days of Ramadan every year. But when the year in which he died, he observed I'tikaf for twenty days.

كَانَ النَّبِيُّ ﷺ يَعْتَكِفُ كُلَّ رَمَضَانَ عَشَرَةَ أَيَّامٍ فَلَمَّا كَانَ الْعَامُ الَّذِي قُبِضَ فِيهِ اعْتَكَفَ عِشْرِينَ يَوْمًا