Creating an incremental job number for each image generated in my app and to log it

Question

My C#.Net WPF app generates an image from user data. It has a max licensed number of images that can be generated. Need to create an incremental job number and display it against the max licensed number. Once max is reached the app should not allow any further generation of images.
Also, the .txt log file needs to display this job number. Any suggestions?