# How many cells/elements do I need?

I'm relatively new in the CFD modelling and I'm making a VOF model of a rectangular box ($$l \times w \times h = 120\times 80 \times8$$ m) with an inlet ($$w\times h = 10\times 8$$) at the long side.

The pressure outlet is the upper face of the box ($$l\times w=120\times 80$$m). The box is initially filled with $$5$$m water. Water is flowing in through the inlet, velocities at inlet: around $$0.1$$ to $$0.01$$ m/s

How many cells/elements do I need to get a good indication for the flow pattern in the box?

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.

Concerning the flow simulation, an upper bound is given by the relation $N\geq Re^{9/4}$, where $N$ is the number of mesh nodes and $Re$ is the Reynolds number, derived for direct numerical simulation .
In your case with $L=120m$, $u=0.1m/s$ and $\nu = 10^{-6}m^2/s$, you have $Re \approx 1.2 \cdot 10^5$ what - in theory - means that you will need about $2.7\cdot 10^{11}$ nodes in your mesh to resolve all scales of the flow dynamics.