To find all the <td> tags which have attribute data-sortable-id you can use the following granular xpath :
"//table[@id='product']/tr//td[@data-sortable-id]"
Perhaps you need to induce WebDriverWait in-conjunction with the expected_conditions clause as visibility_of_all_elements_located as follows :
x = WebDriverWait(webdriver, 30).until(EC.visibility_of_all_elements_located((By.XPATH, "//table[@id='product']//tr//td[@data-sortable-id]")))
print(len(x))