At an evening wedding with dinner and dancing, how do we estimate how
many glasses of wine or beer the average guest will drink?

Just a free comment.

It greatly depends on the crowd: age, drinking habits (which can have
regional/national differences), how lively the party is,  - and how
drinks are served.  The cater/restaurant can push volume by having
staff continually refilling glasses or replacing empty wine bottles on
the tables, also by serving water only when asked to.

You may be able to control this  - if you want to -  by explaining in
advance that after the first two or three rounds you must be asked
each time before further drinks are served, and also insist that water
glasses are refilled or bottles replaced.  If this is at a place with
no service, the same principles apply.

I am not suggesting that you be miserly, just a way to handle things.

One way to find an answer to your question would be to ask a party
restaurant or caterer in your area, one that serves your type of
guests.  (The outfit hosting your party may give you a generous
estimate so that you will be pleased when the costs stay within
budget.)

In an article on planning drinks for a wedding, from the 2005 Bride's
Guide available online from the Bloomington (IN) Herald Times:

Tony Conway, president of legendary events, a catering company in Atlanta
"Assume each adult guest has two drinks during the first hour, and
then tapers off to one drink an hour after that. Some guests will
drink more, some less, but it averages out...." (page 23 of the Guide)

www.heraldtimesonline.com/ htoxtras/bridal/HTBridalGuide.pdf

According to a guide to planning drinks for a wedding put out by the
British Columbia Liquor Board stores, you should estimate the number
of drinking guests and then multiply by 4 for the total number of
servings you will need.

www.bcliquorstores.com/pdf/wedding_planning_guide.pdf 

I think myoarin's comments are all very to the point. The context of
your particular event and the way in which the drinks are
served/managed are significant factors to take into consideration.

Here's a very helpful page that addresses a number of considerations,
as well as provides a way to estimate the quantity of drinks and
applies it with a couple detailed examples:

http://www.montanawines.com/Pages/wines/functions.html