how many musical bands/groups _ participateD in the competition?

+++Tatiana

Russ, that DID is out of place.

can you explain why?

don't we need a help verb here?

No, you use the simple past.

I'm confused a bit. We don't need the help verb here because the question starts with How many? Or there is another reason. I know we don't use help verbs when the question starts with Who (Who sent you there?) But in this case it's How many.
In other hand, there are plenty of examples where I see DO or DID after How many ( how many people did turn out to vote?)

The thing is, “did” CAN be used, but in another context:
A: Most of the contestants were solo singers. A lot of the bands refused to enter the competition.
B: Well how many bands DID participate? (=we know some didn’t, but how many DID?)

But if this contrast isn’t present, then you just say “How many bands participated?”

so, it all about having per-existing contract?
could you come up if a couple of more examples for each case? sometimes it's easier to understand things with more examples.

[Bride’s ex-boyfriend shows up at her wedding and her mother is confused.]
Mother: Oh, hi Jake. How nice to see you. Ummm... did Anne know you were coming??
Jake: Actually, Anne didn’t invite me.
Mother: Well then who DID invite you?
Jake: Nobody! I saw the wedding announcement in the newspaper and had to see with my own eyes the sucker who’s going to marry that cheating daughter of yours!

brutal example ))))
Is it really necessary to use DID in this sentence below?
Mother: Well then who DID invite you?

Can't we just say "then who invited you"?

Yes, but it doesn’t express the contrast: If Anne DIDN’T invite, then who DID invite you?

I see, it's just to express the contract. I wanted to make sure that both versions are ok. Thanks