Here's what we know:
* They were seen by several: The book mentions the car was seen by a policeman, a group of people at a petrol station, and others.
* It was a busy road: They were driving on a "busy trunk road", so there were likely many cars and people around.
* Visibility was good: The car was visible and even attracting attention.
It's safe to assume a fair number of Muggles saw them, but the exact number is left to the reader's imagination.