Necesita mostrar MAX COUNT of Total Customer comparando dos tablas

Al igual que selecciona COUNT(Customer_Nbr) en la 1ª subconsulta, selecciónelo también en la 2ª subconsulta:

SELECT Entity_Name, 
       COUNT(Customer_Nbr) AS `Total Customer`,
       SUM(CASE WHEN Payment_Mode = 'Online Payment' THEN Amount ELSE 0 END) AS `Online Payment`,
       SUM(CASE WHEN Payment_Mode = 'Cash' THEN Amount ELSE 0 END) AS `Cash Payment`,
       SUM(Amount) AS `Total Payment`
FROM mq_paymentlist
GROUP BY Entity_Name

y en la consulta principal use GREATEST() para elegir el valor máximo:

SELECT b.Entity_Name, 
       GREATEST(b.`Total Customer`, p.`Total Customer`) AS `Total Customer`
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