The right number of pictures would be the number that it takes to show the important features. For many listings, 50 pictures would be overkill. The quality of the photographs is more important than the quantity.

One answer might be that it is already sold before officially being entered in the MLS. I agree with you, and put in as many photos as is possible with the listing, knowing that how a listing is portrayed online is the key to bringing potential buyers into the home.

Generally I see it happen due to MLS minimum requirements to enter a listing. We moved from a  maximum of 8 to 26 a couple of years ago. If you can't generate any interest at that point a professional photographer is your best bet.

I believe the minimum requirement is one (1) and the maximum is twenty five (25) ... at least in our Multiple Listing Service.  From my perspective if there is only one picture that tells me the house is not worthy of photos ... however, even on a listing that has no physical merits, if priced right the home should still get the same service and exposure as every listing we carry. I always post the max 25 photos - even if it is mostly exterior shots. My opinion is also that it is a bit of laziness on the agent's part to not give every listing the same shot by providing lots of pictures. With digital cameras there is just no excuse for not giving the buyer lots of pictures to look at.  It does not cost any more money ... not like the good old days of buying film and having it developed.

I also wonder - Why?

Some listing agents do that anyways...

I can only do 17 pictures in my MLS. Although I might take 100 pictures or more. It depends on the property. I list a wide variety of homes. I have custom built homes on acreage with a mountain view to singlewide trailers I list. I will do the same job on photos for all of my listings. If a property is hard to photograph for various reasons, then there will be less pictures. 

As far as pictures in the MLS...well that is a pet peeve of mine. When you have poor or only one picture, that stinks. Is that agent really doing a great job for their client. NO. In my MLS you have to at least have one picture to submit the listing. Some agents will only do that one picture. 

Some are waiting for the photographer to get them finished, others are lazy.

It happens - to differentiate between them and us.

Photos help engage the buyers and good photos make them want to see the property. The more the better.

no idea - more good quality the better

It depends on the type of listing, for lower end homes less is more.