In addition to determining what capacity you need today, assess what capacity you'll need in the future, within a time frame that you can plan for. Growth expectations and changes in how your portal is used are factors you need to accommodate growth.
Develop your growth requirements by answering the following questions:
What is the projected growth for the portal? How fast will the growth occur?
Where is your mobile portal available? What are the trends for use of mobile devices in those countries?
How will your business objectives change in the next two or three years?
What plans do you have for future content?