Look at application/modules/invoices/views/partial_item_table.php
<?php foreach ($items as $item) { ?>
....
<td class="td-amount td-quantity">
<div class="input-group">
<span class="input-group-addon"><?php _trans('quantity'); ?></span>
<input type="text" name="item_quantity" class="input-sm form-control amount"
value="<?php echo format_amount($item->item_quantity); ?>"
<?php if ($invoice->is_read_only == 1) {
echo 'disabled="disabled"';
} ?>>
</div>
</td>
....
<?php } ?>
You should also look at the invoice controller and force the quantity to 1 for each item.