Shaykh al-Islaam Ibn Taymiyyah stated: "...Anyone who assumes that the night prayers of Ramadhan have been limited by the Prophet (may Allaah raise his rank and grant him peace) to a set number that must be adhered to without any increase or decrease has surely erred..."

http://www.bakkah.net/en/taymiyyah-night-prayers-ramadhan-11-20-rakahs.htm