Since microfiche are stored in sheets, it can be difficult to estimate the number of images you have. Understanding the volume of microfiche you need to scan is one of the most important aspects of your project.
One easy way to ballpark the number of images you have is to use a ruler. Create a stack 1 inch thick of individual sheets, and then count the number of sheets x the number of images per sheet. You can use that measurement to get a very basic estimate.