Issue in accessing elements in array of option menu created in python tkinter

OptionMenu executes function with one agument - selected value - so you have to use lambda with two arguments

command=lambda val, n=num:new_number_of_pds(val, n))

First is selected value, second will be optionmenu's number "num".

And then function has to get two arguments

def new_number_of_pds(value, optionmenu_number):

I don't know what you try to do with those elements but if you want to replace item in list then maybe you should create list with 5 empty elements which you can replace with values.

optionmenu number: 0
value: 3
a: ['3', None, None, None, None]
-----
optionmenu number: 2
value: 5
a: ['3', None, '5', None, None]
-----

Code

import tkinter as tk

# --- functions ---

def new_number_of_pds(value, optionmenu_number):
    print('optionmenu number:', optionmenu_number)
    print('value:', value)

    a[optionmenu_number] = value
    print('a:', a)
    print('-----')

def new_number_of_pd(oE_number_of_pd):
    global a

    # delete previous values 
    a = [None, None, None, None, None]

    for num in range(int(oE_number_of_pd)):
        E_Name[num] = tk.OptionMenu(top, b[num], *list2, command=lambda val, n=num:new_number_of_pds(val, n))
        E_Name[num].pack(side="left")

# --- main ---

list2 = ['1', '2', '3', '4', '5']

E_Name=[None, None, None, None, None]

a = [None, None, None, None, None]

top = tk.Tk()
b = [tk.StringVar(), tk.StringVar(), tk.StringVar(), tk.StringVar(), tk.StringVar()]

number_of_pd = tk.Label(top, text="Number of Products")
number_of_pd.pack(side="left")

oE_number_of_pd = tk.StringVar()

E_number_of_pd = tk.OptionMenu(top, oE_number_of_pd, *list2, command=new_number_of_pd)
E_number_of_pd.pack(side="left")

top.mainloop()