Are you talking about the Statistics page ("nnn filled out form")?
What are your SC qualification rules? Can people run through more than once?
Did you verify the number of form fills using a smart list? Or are you just looking at the landing page performance report? Try creating a smart list for all the leads which filled out the form. If you see 478 leads in the smart list, something might be blocking the thanks email to go out. Do a random check for the leads and see if there is anything which does not match the criteria. Also, is your 'Thanks' email an operational email? There might be chances that the communication limit for the leads might be blocking the thanks email.