On average, how many brides are there at a wedding?

One average just one. Some religions do have group marriage ceremonies but at most western weddings there is just one bride and one groom. 

that's easy.  The answer is at least ONE too many...

At most weddings, there is only one bride. 

Well it has to be more then one!

I say that because the largest mass wedding had 40,000 people get married in a single ceremony. That means 20,000 couples and one of them was the bride.

Considering no one knows how many weddings there have been and how many couples got married per ceremony- there is no way to compute it.

The general rule is One Bride per Groom- but even that might not be true! Some men may have married more then one bride at the same time or some women may have married more then one groom at the same time. Also in same sex marriages one of them may not consider themselves the Bride or both may have considered themselves the bride.

So this seemingly simple question is not so simple and our records are such that no one can know.