As far as I know, only the one after the prophet Muhammad's name peace be upon him is obligatory, the rest are preferred to say, out of respect and considered good to say, but not obligatory.
Hmmm I'm kind of doubting myself now, though, is what I'm saying correct?
Three hadiths that explain the importance of saying the salawat when the prophet peace be upon him's name is mentioned:
"The Messenger of Allah said,May he be humiliated, the man in whose presence I am mentioned and he does not send Salah upon me; may he be humiliated, the man who sees the month of Ramadan come and go, and he is not forgiven; may he be humiliated, the man whose parents live to old age and they do not cause him to be granted admittance to Paradise."
"A supplication remains suspended between heaven and earth and does not ascend any further until a person sends Salah on me. Do not treat me like a spare water container, send Salah upon me at the beginning of your supplication, at the end, and in the middle."
"One morning the Messenger of Allah was in a cheerful mood and looked happy. They said, 'Oh Messenger of Allah, this morning you are in a cheerful mood and look happy.' He said, Of course, just now someone [an angel] came to me from my Lord [Allah] and said, 'Whoever among your Ummah sends Salah upon you, Allah will record for him ten good deeds and will erase for him ten evil deeds, and will raise his status by ten degrees, and will return his greeting with something similar to it.'"
