How many people should stand by the Bimah during Kriyah, and when do we need that many people?
Halachically there has to be two people standing by the bimah when he torah is read, therefore if the baal koreh gets the first aliya someone else should be standing by the bimah then. The minhag however is that there should be three people there. The reason is because when the torah is read it is considered a time when H-shem Himself is giving us and telling us the torah, at the time of reading the torah is a mini kabolas hatorah. As a side point This is why it is so disrespectful and such a great sin to talk during Kriyas Hatorah. It is if H-shem in all His loftiness and greatness is talking to us, and people attending the “speech” have the audacity to chat in the middle of it!. When the torah was given there was H-shem giving the Torah, represented by the gabbai who is commanding to read the torah, the baal koreh is in the place of Moshe Rabeinu, and the person getting the aliya represent the people who received the torah- Klal Yisroel.
O:CH 141-4, M:B 141-15,16 quoting Levush.