I used to do ghusl with the intention “I intend to do ghusl” without any specific reason such as janabah, or other reasons for ghusl. I did not know at the time that these ghusl would not be sufficient as Wudhu because I didn’t have a specific reason, so I would go on to pray Fajr with this ghusl on. I do not know how many Fajr prayers I have done improperly because of this issue, but it is at least 20 prayers. I would like to know if I must make these prayers up qada, or I can leave them alone since I did not know at the time my wudhu was insufficient.

If you truly did not know then you do not need to repeat the prayers