Specifying a particular number of times for praying for forgiveness (istighfaar)
Question

                Is it permissible to pray for forgiveness a particular number of times, such as saying it one hundred times today and two hundred times tomorrow?.
Praise be to Allah.It is mustahabb for the 
    Muslim to pray for forgiveness a great deal. The Prophet  (peace and 
    blessings of Allaah be upon him) used to pray for forgiveness one hundred 
    times every day and night. 
It was narrated from 
    al-Agharr al-Muzani (may Allaah be pleased with him) that the Messenger of 
    Allaah  (peace and blessings of Allaah be upon him) said: “I ask Allaah 
    for forgiveness one hundred times a day.” Narrated by Muslim (2702).  
It was narrated from Abu 
    Hurayrah (may Allaah be pleased with him) that the Messenger of Allaah  
    (peace and blessings of Allaah be upon him) said: “I ask Allaah, may He be 
    glorified and exalted, for forgiveness and I repent to Him one hundred times 
    every day.” Narrated by al-Nasaa’i in al-Sunan al-Kubra (6/114) and 
    by Ahmad in al-Musnad (2/450); it was classed as hasan by the 
    commentators on al-Musnad. 
And Abu Hurayrah (may 
    Allaah be pleased with him) said: I heard the Messenger of Allaah  
    (peace and blessings of Allaah be upon him) say: “By Allaah, I ask Allaah 
    for forgiveness and I repent to Him more than seventy times every day.” 
    Narrated by al-Bukhaari (6307). 
It was narrated that Ibn 
    ‘Umar (may Allaah be pleased with him) said: We used to count the Messenger 
    of Allaah  (peace and blessings of Allaah be upon him) saying in a 
    single gathering one hundred times: Rabb ighfir li wa tubb ‘alayya innaka 
    anta al-Tawwaab al-Raheem (O Lord, forgive me and accept my repentance, 
    for You are the Acceptor of repentance, the Most Merciful). Narrated by Abu 
    Dawood (1516) and al-Tirmidhi (3430); classed as saheeh by al-Albaani in 
    Saheeh Abi Dawood. 
Abu Hurayrah (may Allaah be 
    pleased with him) said: I never saw anyone say “Astighfir Allaah wa 
    atoobu ilayh (I ask Allaah for forgiveness and I repent to him)” more 
    than the Messenger of Allaah  (peace and blessings of Allaah be upon 
    him). Narrated by al-Nasaa’i in al-Sunan al-Kubra (6/118). 
If you want to stick to 
    this number (one hundred) following thereby the example of the Prophet  
    (peace and blessings of Allaah be upon him), then you will have the reward. 
    And if you want to do more than that, there is nothing wrong with it so long 
    as you do not choose a specific number or a specific way; rather when doing 
    the additional amount you should do whatever is easy for you, because it is 
    not prescribed to set a specific number for acts of worship if it is not 
    narrated in the shar’i texts. 
And Allaah knows best.