In PE 2018 how do I create a photo collage using only 13 photos?

I'm trying to make a photo collage of the my granddaughter's 1st year. (newborn + every month pictures up to 1st birthday)

All the layouts are a specific number of pictures, none of which are 13.  Can I delete the unwanted photo slots?  If so, how?

Do I have to do this photo collage without using the create a photo collage, in order to only have a specified number of photos?

Thanks