Dear Rabbi, As Bnei Yisrael left Egypt, it is mentioned that a mass of others left Egypt and came with them. In all the counting of Bnei Yisrael and the tribes, there is no mention of them. What happened? Where did they go? Many thanks

I understood your question that you are referring to the "Erev Rav". The Erev Rav are mentioned only once in the entire Torah.(Shmot 12:38) However, in the Midrashic literature they are mentioned many times.(see for example Shmot Rabba 42:6) Their attachment to the Jewish people is viewed negatively by Chazal. Their intentions for joining the Jewish people were not sincere and therefore they were troublesome. They caused the Jewish people to sin with the Golden Calf and were a stumbling block in other trials during the wandering in the desert. It seems that once their negative affect upon the Jewish people became clear they fell into twilight and receive no more attention. However, according to the Zohar there were be traces of the Erev Rav in every generation and will only cease to be whem Mashiach comes.