title: how to count how many times a string occurs in a column using pandas
body: <p>MY DF:</p>
  
  <pre><code>c1 | C2 | C3
  A  | B  | C
  A  | B  | N
  S  | B  | I
  </code></pre>
  
  <p>I want to know how many times <code>B</code> occured in column <code>C2</code>.</p>
  
  <p>I want that output in a list</p>
  
  <p>Desired output:</p>
  
  <pre><code>mylist=[3]
  </code></pre>