Below is an instruction that describes a task. Write a response that appropriately completes the request.

### Instruction:
How many air dates does the episode with 15.50 million viewers have?

### Inputs:
table_name: table, columns: ['No.', 'Title', 'Directed by', 'Written by', 'U.S. viewers (million)', 'Rank (week)', 'Original air date', 'Production code']

### Response:
SELECT COUNT Original air date FROM table WHERE U.S. viewers (million) = 15.50