A grid collage has fixed number of rows and columns. You can increase/decrease the number of rows and columns to suit your collage.
Go to Settings tab in the left side of the application.
In the Layout section, click the up/down arrows to increase/decrease the number of rows/columns in the collage .
Decreasing the number of rows might automatically result in an increase in the number of columns so that there are sufficient number of positions to place all the pictures.