Adding custom button in acumatica

Use custom unbound fields (NonPersistedField) as placeholders to display the totals.

First add those custom fields to SOOrder DAC extension:

Total quantity custom field:

Total transactions custom field:

Create a SOOrderEntry graph extension to compute/update totals:

Add custom fields to Sales Order screen, no need to use label controls, DAC DisplayName property will act as the field label:

Locate the Base graph Transactions DataView which contains the detail data needed for computing the totals (for information only):

In your SOOrderEntry extension use the Transactions DataView from Base Graph to compute totals:

namespace PX.Objects.SO
{
  public class SOOrderEntry_Extension:PXGraphExtension<SOOrderEntry>
  {
      // Initialize unbound values in FieldSelecting events
      public void SOOrder_UsrTotalQty_FieldSelecting(PXCache sender, PXFieldSelectingEventArgs e)
      {
          e.ReturnValue = GetTotalQty(sender);
      }

      public void SOOrder_UsrTotalTransactions_FieldSelecting(PXCache sender, PXFieldSelectingEventArgs e)
      {
          e.ReturnValue = GetTotalTransactions(sender);
      }

      // Update values
      public void SOLine_RowDeleted(PXCache sender, PXRowDeletedEventArgs e)
      {
          UpdateTotals(sender, e.Row as SOOrder, true, true);
      }

      public void SOLine_RowInserted(PXCache sender, PXRowInsertedEventArgs e)
      {
          UpdateTotals(sender, e.Row as SOOrder, true, true);
      }

      public void SOLine_OrderQty_FieldUpdated(PXCache sender, PXFieldUpdatedEventArgs e)
      {
          UpdateTotals(sender, e.Row as SOOrder, true, false);
      }

      public void UpdateTotals(PXCache sender, SOOrder soOrder, bool isUpdateQty, bool isUpdateTranCount)
      {
          // Get SOOrder DAC extension
          if (soOrder != null)
          {
              SOOrderExt soOrderExt = sender.GetExtension<SOOrderExt>(soOrder);
              
              if (soOrderExt != null)
              {
                   // Update values
                   if (isUpdateQty)
                   {
                       soOrderExt.UsrTotalQty = GetTotalQty(sender);
                   }
               
                   if (isUpdateTranCount)
                   {
                       soOrderExt.UsrTotalTransactions= GetTotalTransactions(sender);
                   } 
              }
          }
      }
            
      // Compute totals
      public decimal? GetTotalQty(PXCache sender)
      {
          decimal? totalQty = 0M;

          // Compute quantity from SOOrderEntry base graph Transactions DataView
          foreach (SOLine soLine in Base.Transactions.Select())
          {
              totalQty += soLine.OrderQty;
          }

          return totalQty;
      }

      public int? GetTotalTransactions(PXCache sender)
      {
          return Base.Transactions.Select().Count();
      }
  }
}

Totals are displayed in Sales Order screen, they will update when inserting/removing Transaction rows and modifying order quantities: