Calculating percentage of count and adding it to the count in MySQL table with the query
For general case use your original query as derived table:

SELECT cnt + (cnt * 0.75) AS total
FROM (
    SELECT COUNT(order_id) AS cnt
    FROM orders
    WHERE order_date BETWEEN CURDATE() + ' 00:00:00' AND NOW()
) AS tmp


But in your case it is much simpler:

SELECT (COUNT(order_id) * 1.75) AS total
FROM orders
WHERE order_date BETWEEN CURDATE() + ' 00:00:00' AND NOW()