iis server sizing requirement for 500 users

we need tp support around 5000 users which are on IIS ... with some front end application running out of IIS and backend on some kind of DB like SQL or oracle ..

i am not able to find out .. what and how much hardware i need .. like configuration of RAM.. cpu .. Space ... and bandwidth ..

is there anything.. like calculation or some best practicles which can guide me in this .. i wont mind dividing these 5000 users by 10 servers ,.. so it should not go all on one server .. if 5000 is a big number.. and most probably these will be VMs...