Assuming no edge effects/turbulence, we can calculate the force of wind on a single sail using the drag equation.
$$\vec{F_D} = \tfrac12 \rho \vec{u}^2 C_D A$$
where the drag coefficient $C_D$ is going to be fairly high, for a concave sail.
In general, however, the wind may not be hitting the sail orthogonally, but rather at some angle. We can generalise the drag equation as such:
$$\vec{F_{sail}} = \tfrac12 \rho \vec{u}^2 C_D (\hat{\vec{u}} \cdot \vec{S})$$
where $S% is the vector area of the sail. We take the dot product of this vector with the unit velocity vector to resolve the wind force in the direction of the sail. This is of course an approximation, as the concave shape of the sail will come into play, but probably a good enough one.
That pretty much explains the basic situation for a single sail. Now, for multiple sails, you would of course simply combine the forces on the individual sails, which will all have their own values of $C_D$ and $S$.
$$\vec{F_{sails,total}} = \sum \vec{F_{sail}}$$
I suggest you familiarise yourself with the mechanics and specific equations used here, and use that as a basic model to start.
For some general information on the physics of sailing, you may find this page helpful.