Is there a specific number of Rak`ahs for optional Qiyaam al-Layl

Question 17:  Is it necessary to offer a specific number of Rak`ahs (units of Prayer) for Qiyaam-ul-Layl (standing for optional Prayer at night) every night or is it done according to one’s ability?

Answer: It is prescribed for a Muslim to offer eleven Rak`ahs on standing for optional night Salaah (Prayer), as this was the number mostly observed by the Prophet (peace be upon him). It was authentically reported that `Aa’ishah (may Allaah be pleased with her) said... read more here.