The first question to be answered is "How many images are needed to complete the mosaic?" The answer to this question helps you gauge how much work is going to be required in terms of both image acquisition and image processing. If the number is too high (say over 50) you might want to consider a shorter focal length (or a larger camera). On the other hand if you are up to the task there really is no limit to the number of lunar images that can be stitched together into a mosaic. The number of images needed is a function of:

You can calculate the field of view of your imaging device using this formula:

The detector size and focal length are specified in millimeters and the field of view is given in arc minutes. Finally, take into consideration the amount of image overlap. To ensure no gaps are present in the completed mosaic you should overlap your image segments in amounts of 15% to 25%. It is better to err on the side of too much than too little. Subtract this percentage from the field of view. Assuming a 20% overlap and a full lunar disk, the number of images in the x and y dimension then becomes a simple calculation:

Round these numbers up to the next whole number and multiply together to get a good starting estimate. As an example, consider an SBIG ST7 CCD camera on a Celestron C8. That's a focal length of about 2000mm and an image size of 6.9mm by 4.6mm. If you do the math you will see that it takes 20 images to capture the entire disk of the full Moon.