You are now a sqlite data analyst, and you are given a database schema as follows:

    【Schema】
    【DB_ID】 car_retails
【Schema】
# Table: products
[
(productCode:TEXT, Primary Key, Examples: [S10_1678, S10_1949, S10_2016]),
(productName:TEXT, Examples: [1969 Harley Davidson Ultimate Chopper]),
(productLine:TEXT, Examples: [Motorcycles, Classic Cars, Trucks and Buses]),
(productScale:TEXT, Examples: [1:10, 1:12, 1:18]),
(productVendor:TEXT, Examples: [Min Lin Diecast]),
(productDescription:TEXT),
(quantityInStock:INTEGER, Examples: [7933, 7305, 6625]),
(buyPrice:REAL, Examples: [48.81, 98.58, 68.99]),
(MSRP:REAL, Examples: [95.7, 214.3, 118.94])
]
# Table: orderdetails
[
(orderNumber:INTEGER, Primary Key, Examples: [10100, 10101, 10102]),
(productCode:TEXT, Primary Key, Examples: [S18_1749, S18_2248, S18_4409]),
(quantityOrdered:INTEGER, Examples: [30, 50, 22]),
(priceEach:REAL, Examples: [136.0, 55.09, 75.46]),
(orderLineNumber:INTEGER, Examples: [3, 2, 4])
]
# Table: payments
[
(customerNumber:INTEGER, Primary Key, Examples: [103, 112, 114]),
(checkNumber:TEXT, Primary Key, Examples: [HQ336336, JM555205, OM314933]),
(paymentDate:DATE, Examples: [2004-10-19]),
(amount:REAL, Examples: [6066.78, 14571.44, 1676.14])
]
# Table: orders
[
(orderNumber:INTEGER, Primary Key, Examples: [10100, 10101, 10102]),
(orderDate:DATE, Examples: [2003-01-06]),
(requiredDate:DATE, Examples: [2003-01-13]),
(shippedDate:DATE, Examples: [2003-01-10]),
(status:TEXT, Examples: [Shipped, Resolved, Cancelled]),
(comments:TEXT),
(customerNumber:INTEGER, Examples: [363, 128, 181])
]
# Table: productlines
[
(productLine:TEXT, Primary Key, Examples: [Classic Cars, Motorcycles, Planes]),
(textDescription:TEXT),
(htmlDescription:TEXT),
(image:BLOB)
]
# Table: employees
[
(employeeNumber:INTEGER, Primary Key, Examples: [1002, 1056, 1076]),
(lastName:TEXT, Examples: [Murphy, Patterson, Firrelli]),
(firstName:TEXT, Examples: [Diane, Mary, Jeff]),
(extension:TEXT, Examples: [x5800, x4611, x9273]),
(email:TEXT),
(officeCode:TEXT, Examples: [1, 6, 4]),
(reportsTo:INTEGER, Examples: [1002, 1056, 1143]),
(jobTitle:TEXT, Examples: [President, VP Sales, VP Marketing])
]
# Table: offices
[
(officeCode:TEXT, Primary Key, Examples: [1, 2, 3]),
(city:TEXT, Examples: [San Francisco, Boston, NYC]),
(phone:TEXT, Examples: [+1 650 219 4782, +1 215 837 0825, +1 212 555 3000]),
(addressLine1:TEXT, Examples: [100 Market Street, 1550 Court Place, 523 East 53rd Street]),
(addressLine2:TEXT, Examples: [Suite 300, Suite 102, apt. 5A]),
(state:TEXT, Examples: [CA, MA, NY]),
(country:TEXT, Examples: [USA, France, Japan]),
(postalCode:TEXT, Examples: [94080, 02107, 10022]),
(territory:TEXT, Examples: [NA, EMEA, Japan])
]
# Table: customers
[
(customerNumber:INTEGER, Primary Key, Examples: [103, 112, 114]),
(customerName:TEXT, Examples: [Atelier graphique]),
(contactLastName:TEXT, Examples: [Schmitt, King, Ferguson]),
(contactFirstName:TEXT, Examples: [Carine , Jean, Peter]),
(phone:TEXT, Examples: [40.32.2555, 7025551838, 03 9520 4555]),
(addressLine1:TEXT, Examples: [54, rue Royale, 8489 Strong St., 636 St Kilda Road]),
(addressLine2:TEXT, Examples: [Level 3]),
(city:TEXT, Examples: [Nantes, Las Vegas, Melbourne]),
(state:TEXT, Examples: [NV, Victoria, CA]),
(postalCode:TEXT, Examples: [44000, 83030, 3004]),
(country:TEXT, Examples: [France, USA, Australia]),
(salesRepEmployeeNumber:INTEGER, Examples: [1370, 1166, 1611]),
(creditLimit:REAL, Examples: [21000.0, 71800.0, 117300.0])
]
【Foreign keys】
products.productLine=productlines.productLine
orderdetails.productCode=products.productCode
orderdetails.orderNumber=orders.orderNumber
payments.customerNumber=customers.customerNumber
orders.customerNumber=customers.customerNumber
employees.officeCode=offices.officeCode
employees.reportsTo=employees.employeeNumber
customers.salesRepEmployeeNumber=employees.employeeNumber

    【Question】
    How many customers have an employee who reports to William Patterson as their sales representitive? reportsTO' is the leader of the 'employeeNumber';

    【Evidence】
    reportsTO' is the leader of the 'employeeNumber';

    Please read and understand the database schema carefully, and generate an executable SQL based on the user's question and evidence. 

SQL: SELECT COUNT(t1.customerNumber) FROM customers AS t1 INNER JOIN employees AS t2 ON t1.salesRepEmployeeNumber = t2.employeeNumber WHERE t2.firstName = 'William' AND t2.lastName = 'Patterson'