I'm relatively new in the CFD modelling and I'm making a VOFmodel of a rectangular box (lxwxh 120x80x8m) with an inlet (wxh 10x8) at the long side. The pressure outlet is the upper-face of the box (lxw 120x80m)Box is initially filled with 5m water.Water is flowing in through the inlet, velocities at inlet: around 0.1 to 0.01 m/sHow many cells/elements do I need to get an good indication for the flow pattern in the box? 
How many cells/elements do I need?
mesh generation;elements
The answer to your question can depend on quite a few things, such as whether you need a turbulence model, and which one you choose if so, and how you are handling the top of your fluid since moving interfaces are nontrivial.  You are also imposing discontinuities on the boundary at the edges of your inlet so a nonuniform mesh would be advantageous for you.  While someone may be able to give you a real answer, the best way is to find it yourself.  Mesh your problem with what you deem a reasonable mesh, solve it, mesh it again with twice as many points, and see if your answers changed.  If they didn't you are done, and you have your answer.  If they did, either double your number of points again if you think you are close, or start over with the meshing based on where you biggest errors are.  Not only will this guarentee a good answer for you problem, it will also start to build your intuition for what types of features need more mesh points, so the next time you will start with a much better mesh.