Think of it.
Imagine, you need only one loop. Based on your example - listbox1 has got 1000 items. Then you want to split it into 5 listboxes.
int j = 0;
for(int i=0; i<1000; i++)
{
j = i % 5;
Console.WriteLine($"Item {i} goes to the listbox {j}");
}
Result:
Item 0 goes to the listbox 0
Item 1 goes to the listbox 1
Item 2 goes to the listbox 2
Item 3 goes to the listbox 3
Item 4 goes to the listbox 4
Item 5 goes to the listbox 0
Item 6 goes to the listbox 1
Item 7 goes to the listbox 2
Item 8 goes to the listbox 3
Item 9 goes to the listbox 4
Item 10 goes to the listbox 0
Item 11 goes to the listbox 1
...
Item 994 goes to the listbox 4
Item 995 goes to the listbox 0
Item 996 goes to the listbox 1
Item 997 goes to the listbox 2
Item 998 goes to the listbox 3
Item 999 goes to the listbox 4
You can also split items different way. 200 first items can go into listbox #1, next 200 items can go into listbox #2, etc. You need to modify above code to your needs.
Good luck!
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