Was Elisha alone able to plow with 12 Tzemadim (yoked oxen)?
Radak: He plowed with his servants.
How many oxen are in 12 Tzemadim?
Radak: There were 12. Oxen yoked at the time of plowing are called Tzemadim.
Radak citing Targum Yonasan: There were 12 pairs of oxen.
What is the meaning of "v'Hu bi'Sheneim ha'Asar"?
Radak citing Targum Yonasan: He plowed with one of the 12 Tzemadim (and 11 servants with the others).
Malbim: He plowed with the last one. (And even so, he ran after Eliyahu 1 .)
I.e. even though there was no one after him to oversee the Tzemed with which he plowed; it could go away and get lost. (PF)
Why did the Navi teach that there were 12 Tzemadim?
Radak: This was a Siman that he will be appointed over the 12 Shevatim, to be a Navi and to warn them.
Malbim: Refer to 19:20:2:3.
Why does it say "va'Yashlech Adarto Elav"?
Radak citing Targum Yonasan: This is like Alav.
Why did he cast his cloak on him?
Radak (19) Eliyahu did not leave his cloak on him; he took it back. This was a Siman that Elisha should follow him (refer to 19:20:1:1). He did not tell him explicitly; he knew that Elisha will understand that Hashem wants to him to be a Navi, and Hashem put in his heart to go after Eliyahu and serve him.
Malbim: This was a Siman that Elisha will follow him and take his cloak (position).
