If I joined the Imam in praying and made up the missed rak’as in prayer but in the meantime I forgot
how many rak’as I have prayed, am I required to do prostration of forgetfulness [Ar.Sujud al-Sahw] ?

In this case, you must decide by undertaking the most likely assumption. If the two assumptions you have carry the same weight, you should follow the least assumption, prostrate for Sahw (forgetfulness) before ending the prayer with salam. Please note that prostration for sahw is sunna.