Congratulations on 20 years of marriage! =D
Although it may seem strange to you or I, they DO like wedding photos.
It's always better to send a good quality sampling of your time together, for instance in your case a photo or two from before marriage, wedding photo (1 or 2) and a pic from each 'major' event. As for the holidays: if you celebrate Christmas, let's say, you'd send three or four from various years, not all 20 years. Same with other major celebrations. (I use Christmas as an example; your culture/religion may have Kwanzaa, Passover, etc.)
Some say, 'more is better'. Let a discerning eye be the judge. It's better with perhaps 40 - 50 pictures from all 20 years picking out the 'good stuff' than 2 000 pictures from each year just because you both are shutterbugs. ^^ Try to go for a storyboard - you're telling your love story with the photos, covering a span of time that shows solidity and comprehension in a relationship. If a stranger can /see/ the progression of time in your relationship, you've done it right.
Bidding you all the best wishes!