How to Count Occurrences in a Column while including another column with unique values with SQL?

I have a table of customers:

I want to query for all customers, and an additional column that counts how many customers are of the same order number:

I have tried this syntax

SELECT A.id, A.0rder, B.cnt
  FROM Customers  A
  INNER JOIN (
    SELECT Order, count(Order) as cnt
      FROM Customers 
      GROUP BY Order
  ) B ON A.Order = B.Order

and ran into this error. 'Error saving the Query field.Old style JOIN (ANSI JOINs) syntax is not allowed. Please use standard syntax.'

The issue with your query is that your column named order is also a keyword eg ORDER BY. You may resolve this by using a table alias as shown below:

SELECT 
    A.id, A.order, 
    B.cnt 
FROM 
    Customers A 
INNER JOIN (
  SELECT b.Order, count(b.Order) as cnt 
  FROM Customers b GROUP BY b.Order
) B ON A.Order = B.Order;

View on DB Fiddle

You may also use a window function COUNT with a partition on the order column or use a correlated query to achieve this. Both approaches have been included below.

Query #1 - Window Functions

SELECT
    t1.*,
    COUNT(1) OVER (PARTITION BY t1.order) as count
FROM
    my_table t1
ORDER BY 
    t1.id;

Query #2

SELECT
    t1.*,
    (
        SELECT COUNT(1) FROM my_table t2 WHERE t2.order=t1.order
    ) as count
FROM
    my_table t1
ORDER BY 
    t1.id;

View Demo on DB Fiddle

Let me know if this works for you.