Sorry but your question is far too vague and the amount of servers can be wide ranging.
If you have never installed then it probably best attempting it on one server first before moving on to a distributed install
As both have said I'd suggest you to have 3 servers. 3 if you only go one sql/ oracle box, one Essbase server, and one java server (planning and reports). Assuming you have the resources. If not I would look towards a hosted option it's cheaper and less hardware costs involved.