Q:

The quantity of items after executing a test not matched with expected quantity

I have an user (administrator) already existed in database.  I run the cucumber test and create another user.  I expected the 2 users after executing a scenario, but got 1.  Where is my mistake?  May be cucumber does not count an existing entities in db if they are not created via cucumber...

A:

Your test database is cleared before every test.  You have to create the Admin as part of your Cucumber steps, probably with a "Background:" is the best way. 
edit To log in as an admin, do something like this:
Background:
  Given I am an admin
  And I have logged in

  Scenario: Create a user
    Given I go to the new user page
    And I fill in "email" with "test@user.com"
    And I submit the form
    Then there should be 2 users

Hopefully you're able to write those step definitions yourself. " Given I am an admin" will just create an admin user, and "I have logged in" will go through the Login page, logging in as the admin.