Please read the following table in Markdown format and then answer the question according to the table. Table cells in one row are seperated by '|', and different rows are seperated by '
'.

Table:
| Season | Episodes | Time slot (EST) | Season premiere | Season finale | TV season | Rank | Viewers (in millions) |
| --- | --- | --- | --- | --- | --- | --- | --- |
| 1.0 | 23.0 | Wednesday 10pm/9c | September 22, 2004 | May 18, 2005 | 2004–2005 | #21 | 13.59 |
| 2.0 | 24.0 | Wednesday 10pm/9c | September 28, 2005 | May 17, 2006 | 2005–2006 | #22 | 14.04 |
| 3.0 | 24.0 | Wednesday 10pm/9c | September 20, 2006 | May 16, 2007 | 2006–2007 | #25 | 13.92 |
| 4.0 | 21.0 | Wednesday 10pm/9c | September 26, 2007 | May 21, 2008 | 2007–2008 | #28 | 11.71 |
| 5.0 | 25.0 | Wednesday 10pm/9c | September 24, 2008 | May 14, 2009 | 2008–2009 | #17 | 13.50 |
| 6.0 | 23.0 | Wednesday 10pm/9c | September 23, 2009 | May 26, 2010 | 2009–2010 | #23 | 12.66 |
| 7.0 | 22.0 | Friday 9pm/8c | September 24, 2010 | May 13, 2011 | 2010–2011 | #37 | 10.73 |
| 8.0 | 18.0 | Friday 9pm/8c | September 23, 2011 | May 11, 2012 | 2011–2012 | #38 | 10.34 |
Question:
How many season premiers have a rank of #21?
Answer:
September 22, 2004