2082. The Number of Rich Customers

Table: Store

+-------------+------+
| Column Name | Type |
+-------------+------+
| bill_id     | int  |
| customer_id | int  |
| amount      | int  |
+-------------+------+
bill_id is the primary key (column with unique values) for this table.
Each row contains information about the amount of one bill and the customer associated with it.

Write a solution to report the number of customers who had at least one bill with an amount strictly greater than 500.

The result format is in the following example.

Example 1:

Input: 
Store table:
+---------+-------------+--------+
| bill_id | customer_id | amount |
+---------+-------------+--------+
| 6       | 1           | 549    |
| 8       | 1           | 834    |
| 4       | 2           | 394    |
| 11      | 3           | 657    |
| 13      | 3           | 257    |
+---------+-------------+--------+
Output: 
+------------+
| rich_count |
+------------+
| 2          |
+------------+
Explanation: 
Customer 1 has two bills with amounts strictly greater than 500.
Customer 2 does not have any bills with an amount strictly greater than 500.
Customer 3 has one bill with an amount strictly greater than 500.