The first control, Number of images to resample in parallel, is a control on speed. You want this number to be as large as possible without running out of memory. For example, if you have 5MB images, then running Idealize with 10 images in parallel means that 50MB of memory will be needed. The Idealizer will load 10 images, idealize them, close them and then load the next 10 images.
Note: The default value shown in this control will be the lower of the number of images in your project and the maximum allowed based on the image size and available RAM in your computer.