Among the menu pages on which the dish "Paysanne Soup" had appeared, how many of them had a stable price for the dish?
Paysanne Soup is a name of dish; stable price refers to highest_price is null;
SELECT SUM(CASE WHEN T1.name = 'Paysanne Soup' THEN 1 ELSE 0 END) FROM Dish AS T1 INNER JOIN MenuItem AS T2 ON T1.id = T2.dish_id WHERE T1.highest_price IS NULL