What are the conditions for tayammum, and how many prayers can be performed with a single tayammum in the absence of water or fear of illness?

For the validity of tayammum, it is required that the time for prayer has begun, that water is unavailable or its use is hindered due to illness, and that one seeks water if it is absent. The Hanafis and Ibn Taymiyyah held that one who performs tayammum may pray as many obligatory and supererogatory prayers as he wishes with that tayammum, whereas the Malikis, Shafi'is, and Hanbalis believe that one must perform tayammum for each obligatory prayer.