He assumed that he had done less in his prayer, but he only did the two prostrations of forgetfulness without doing the rak‘ahs that he had omitted
Question

                I had a mistaken understanding of the prostration of forgetfulness. I used to assume that I had done the lower number of rak‘ahs, but I did not do an additional rak‘ah; I only did the prostration of forgetfulness. This happened to me many times but I do not remember how many. Do I have to do anything?.
Praise be to Allah.
      Firstly: 

    If the worshipper is not sure how many rak‘ahs of the prayer 
    he has done, he should assume it is the lower number and complete his prayer 
    on that basis, then do the prostration of forgetfulness before saying the 
    tasleem. If he is not sure during the rak‘ah whether it is the first and 
    second, he should regard it as the first and complete his prayer on that 
    basis, praying three rak‘ahs after that, then he should do the prostration 
    of forgetfulness before saying the tasleem. 

    See the answer to question no.
    33624. 

    Secondly: 

    If a Muslim offers a prayer that is not correct because he 
    does not know the rulings on prayer, and he continues to do that for a 
    while, then the correct scholarly view is that he does not have to make up 
    past prayers. Rather he should only repeat the last prayer if there is still 
    time for it and its time has not ended. 

    The evidence for that from the Sunnah is the hadeeth of the 
    man who used to pray badly and did not do it at a measured pace. The Prophet 
    (blessings and peace of Allah be upon him) did not instruct him to make up 
    his past prayers; rather he only instructed him to repeat the last prayer. 

    Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) 
    said: 

    He (blessings and peace of Allah be upon him) did not 
    instruct him to repeat whatever prayers he had offered before that, even 
    though he said: I cannot do better than that. Rather he instructed him to 
    repeat that particular prayer because there was still time left for it, and 
    he was enjoined to offer the prayer during its time. That was the prayer 
    that he had not offered properly and there was still time for that prayer. 
    It is well known that if a child reaches puberty or a kaafir becomes Muslim 
    or a menstruating woman becomes pure or an insane person comes to his 
    senses, and there is still time left for a prayer, they should offer it, 
    because that is the right time for it, and it is not to be regarded as 
    making it up. But if that happens after the time for a prayer has ended, 
    there is no sin on them. When this ignorant person who had prayed badly came 
    to know during the time for prayer that it was obligatory to pray at a 
    measured pace, praying at a measured pace became obligatory upon him at that 
    time, but it had not been obligatory for him before that. Hence (the Prophet 
    (blessings and peace of Allah be upon him) instructed him to pray at a 
    measured pace in the prayer that was due at that time, and not those that he 
    had done before it. End quote. 

    Majmoo‘ al-Fataawa, 22/44 

    Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said: 

    If a person omits an obligatory duty because he is unaware 
    that it is obligatory, he is not obliged to make it up if the time for it 
    has ended, based on the fact that the Prophet (blessings and peace of Allah 
    be upon him) did not instruct the one who prayed badly – and did not pray at 
    a measured pace – to make up past prayers. Rather he instructed him to offer 
    the prayer that was currently due in the prescribed manner. End quote. 

    Al-Usool min ‘Ilm al-Usool, p. 
    32 

    Based on that, you do not have to make up missed prayers but 
    you have to pray properly in the future. 

    And Allah knows best.