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

### Instruction:
How many different series numbers are there for the episode seen by 7.84 million people in the US?

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

### Response:
SELECT COUNT No. in series FROM table WHERE U.S. viewers (millions) = 7.84