If you have less than 250 points in your account (which is the free processing threshold) jobs will be limited to 250 points or less until you purchase more processing points.
We charge by the total number of megapixels uploaded. 1 point is enough to process 5 megapixels of imagery. 250 points is enough to process 1250 megapixels. If you are uploading 20 MP images and don't have more than 250 points then you will be able to upload up to 62 images. (1250 / 20 = 62.5)
Step 4 of the uploading workflow tells you this: