Such a simple, but yet, not intuitive and unexpected way to say it!
For "many of us" we say, literally, "many from us!"

Let's see some examples:
⮞ Many of us prefer not to eat alone, if we have a choice. רבים באיתנו מעדיפים לא לאכול לבד, אם יש בחירה (audio #1)
⮞ Many of us do not know how to deal with this problem. רבים מאיתנו לא יודעים איך להתמודד עם הבעיה הזאת (audio #2)
⮞ Many of us believe that bats are blind, but it's not true - bats have eyes! רבים מאיתנו מאמינים שעטלפים הם עיוורים, אבל זה לא נכון - לעטלפים יש עיניים!! (audio #3)

But not only is the preposition different from the one we use in English, but in itself, as far as Hebrew is concerned, it's inconsistent!

Let's inspect the "from" preposition -
⮞ I am from Australia. אני מאוסטרליה (audio #4)
⮞ This present is from me. המתנה הזאת ממני (audio #5)
⮞ It has been a while since I heard from you. מזמן לא שמעתי ממך (audio #6)

So why מאיתנו?!
Why not say ממנו?!

Short answer - it's an option. Yes, you can say ממנו. (Pronounced as "mimenu")
One possible answer for having this alternative form is that saying ממנו is confusing, because it's exactly how we say "from him" - the two forms totally coinside... So I, personally, prefer to say מאיתנו, to avoid confusion. :)

(Here is a relatively short and comprehensive article from the Academy (in Hebrew!) that addresses this question - https://hebrew-academy.org.il/2018/10/10/%d7%9e%d7%9e%d7%a0%d7%95%d6%bc-%d7%90%d7%95-%d7%9e%d7%90%d7%99%d7%aa%d7%a0%d7%95/ )