This work.
Just make a change handler in jquery and prove if it is checked or not.
Check it in the Developer Tools.

$('input[type="checkbox"]').change(function() {
  if (this.checked) {
    $(this).val(1);
  } else {
    $(this).val(0);
  }
});

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<div>
  <input name="need_a" type="checkbox" class="custom-control-input" value="1" checked>
  <label for="scales">Scales</label>
</div>

<div>
  <input name="need_a" type="checkbox" class="custom-control-input" value="1" checked>
  <label for="horns">Horns</label>
</div>

<div>
  <input name="need_a" type="checkbox" class="custom-control-input" value="1" checked>
  <label for="Guitar">Guitar</label>
</div>

Your approach is incorrect, what you need is to create a hidden input with the default value for the unchecked fields an keep the grouped by name, see my example below:

<input name="need_a" type="hidden" value="0">
<input name="need_a" type="checkbox" class="custom-control-input" value="1" {% if some_condition %}checked{% endif %}>

<input name="need_b" type="hidden" value="0">
<input name="need_b" type="checkbox" class="custom-control-input " value="1" {% if some_condition %}checked{% endif %} />

<input name="need_c" type="hidden" value="0">
<input name="need_c" type="checkbox" class="custom-control-input" value="1" {% if some_condition %}checked{% endif %} />

And in django you need to create an array to respect your format such as:

formValues = [request.POST.getlist('need_a'), request.POST.getlist('need_b') , request.POST.getlist('need_c')]

Then you’ll get [1,0,1] / [0,0,1] / etc values.

This is what the browser is sending to django (need_a checked, need_b not checked, need_c checked):

need_a  […]
0   "0"
1   "1"
need_b  "0"
need_c  […]
0   "0"
1   "1

And this is what my backend(PHP) caught on the request:

Array ( [need_a] => 1 [need_b] => 0 [need_c] => 1 ) 

Here I have multiple checkbox with same name. Checkbox can be added to n no. of times.

Here if already checked I want to send 1 and if not 0.

Right now in my django backend I am getting the list with request.POST.getlist('need_a') like this.

If all checked [1, 1, 1] which is fine .

if only one checked then I am getting [1] instead of [1, 0, 0]

 <input name="need_a" type="checkbox" class="custom-control-input" 
 value="1" {% if some_condition %}checked{% endif %}/>

 <input name="need_a" type="checkbox" class="custom-control-input "
 value="1" {% if some_condition %}checked{% endif %}/>

 <input name="need_a" type="checkbox" class="custom-control-input" 
 value="1" {% if some_condition %}checked{% endif %}/>