Schedule system

Need an excel spreadsheet that does the following: I have a list that shows the total number of projects (y), start month (x) and a start number (n), 6 months after x, n is removed from the list (y). If I start 2 projects in July, they will be completed in January. Then I have 1 project that starts in August, which will be completed in February. I have 3 total projects in September. Hope this makes sense. I need a spreadsheet that will allow me to forcast 24 months of projects. Some months might have more than 1 project, which means 6 months from the start date, more than 1 will be complete.