"How many viewers WATCHED the tennis final?"
"How many viewers DID WATCH the tennis final?" However, the normal form would be the first. I would use this one for emphasis- for example to get the correct information after being told that my figure was wrong.
You should distinguish which part of the sentence your question refers to.If it refers to the subject, you don't need an auxiliary in the Present and Past simple. 10,00o fans watched the finals.
The subject is 10,000 fans. Who watched the finals?
How many fans watched the finals?
I read books by H.Wells. What books did you read?
This questions refers to the object, not the subject.
Theoretically you can use do,does,did in questions to the subject, but only emphatically. It's not common enough to be taught to beginners.