While the obvious answer is 2 * Number of people - and this is a good starting point, it may make more sense to be driven by the layout and dimensions of the room.
If money is no issue, If its office space I'd be inclined to work out how many desks I can put in and then cables and (dual) power points for each place a desk can go to allow for future expansion.
If its residential (unlikely here I guess) or money is an issue, I'd put 4 jacks on each wall (presumably 16 in total) and run switches and cables as required - if your network is complex you will need to weigh up the cost/benefit and reliability issues related to running multiple switches - and if your phones are PoE powered you will want to take that into account as well (A single PoE switch is easier to manage and cheaper then deploying small switches and added PSU's - and not all VOIP phones have power supplies (for example Polycoms its an added extra).
A third way of gripping the problem is to mirror the number of power jacks with ethernet cables (but leave a bit of space between the power jacks and ethernet cables - you don't want to run the cables together). This would work on the assumption that you will be limited by your power cables.