It says up to 20 pictures so theoretically you could send none, but it won't help your case. if you can't get to 20, I would supply a well-written note in the bundle stating exactly why there aren't that many photos and try to address any concerns that the visa officer might have. Point out that the photos you did provide show your relationship, and describe how they did.
Though if you are married, it may be difficult to justify not having 20 photos if you had a wedding celebration. But even that may be culture or couple specific.
We had a smaller private ceremony due to travel and health issues for my relos back in Australia, so not many photos. And for sure, we're addressing all this in a letter and part of the space they have on the forms.
We only just got married, so we don't have a ginormous history and a lifetime of photos to pick from. That's all.