As suggested, it's not clear what problem you actually want us to help you with. If you have quantities for 1 item and you want N items then it's fairly obvious that you need to multiply all your quantities by N. Are you really asking us how to perform a multiplication in C#? I would hope not. If you are then you it sounds like what you need to do instead is find a beginners tutorial and work your way through that to learn the basics first.

It sounds like you may be making the classic beginner mistake of treating a programming problem as a monolith. Your application is not one problem. It's a branching series of many much smaller problems. You need to stop and think about the process involved in getting from A to B, not just about A and B themselves. Think about the steps you would perform if this was a purely manual process. Break those steps into smaller and smaller parts until you can't break them down any further. Now treat each part individually and research how to implement that in code. If you can implement each part then you can do the whole thing, because the whole thing is just the sum of the parts, with perhaps a little massaging at the edges. If you can't do a particular part then you can ask us about that part specifically instead of asking us to implement an entire application for you.