Pick the most expensive servers/databases/storage and increase the number of users. Every 1-2 changes in orders of magnitude of the latter will require you to think thoroughly about how to best use resources and not explode your budget.

Also read from the highscalability blog, there's a lot of experience recorded there. See for example this transcript of an AWS presentation about a step-by-step guide to scaling: http://highscalability.com/blog/2016/1/11/a-beginners-guide-.... Of course it's done with AWS bricks but the ideas are universal