1000-1200 concurrent users doing what? I'm afraid there is no way to answer this question. You need to go back to your boss and find out a LOT more information. What are the servers going to do? What sort of transactional load are you facing? What's the expected data load - how much new data do you expect every day, every week? What's your budget? Is failover important? How critical is the data? What are the security requirements?

Find out from your app developers and your boss what it is they actually want, before you try and work out what it is you need.