As someone who follows both 4 and 2 wheels on the ovals, I thought it worth responding to this.
The meeting at Mildenhall was actually Bangers, not Stock Cars. They are very different formulas and different types of racing.
Attendances vary depending on the formulas racing and the level of the meeting. On average probably very similar crowds to Speedway, but some meetings can fill a stadium.
This particular meeting was far from 'run of the mill'. Although it wasn't a major title race it was one that attracted attention due to the type of meeting it was with all the cars being pre-1990. It was a 2 day meeting with over 300 cars booked in, giving fans a chance to see rare cars being raced, stay overnight at the track for a good social, and the weather was superb for the time of year.
With regards the comment from someone else about Odsal looking empty. It has a capacity of over 20,000 so even a 4-5,000 crowd makes the place look sparsely populated. For some of the less popular meetings they only open the stand on the home straight (which I believe holds around 3,000) as this reduces stewarding costs. A youtube video shot from that stand would then obviously show very empty terracing.