Re: How many ?
Somewhat challengely put, I can see you are in management.
So tell me one thing - are you running in normal mode your systems at 100% capacity that you need add more systems at peak load ?
Sort of, they will be running above 50% I guess, and yes load at all tiers of the application will go up, asymmetrically of course because it depends on which of the examples I have given.
How about databases?
We will do that depending on the task and the architecture of the application,and add more stuff to existing db servers
Are you also spawning new instances just for peak load ?
Well peak, mmmm, depends what you mean. Some of the above examples do not arrive at a pointy peak as characterised by the Matterhorn shape. Many are more Aconcagua in form. So we will add to respond to expected loads and try and add quickly if odd stuff is detected. Sometimes the extra is left in place for days as the demand goes on and on.
I am interested in what answers you were expecting? Do these match with your experience in managing variable loads?
All the above is helped by having your software designed at least in part to be able all these shenanigans to happen.
The ERP system I am referring to is stateless and asynchronous and so can scale up and out without batting an eyelid.
Please answer, as I have spent time writing this and I had paint to watch dry