Via Current Rating

A general rule of thumb for typical via's (say 0.5mm) passing high currents between layers is to allow 0.5A nominal per via.  Bigger hole sizes increase this rating with the larger surface area of the plating through the hole, but for high current connection more smaller vias can beneficial if your copper tracks / plane is wide.

Multiple Vias

It's best to spread the stitching vias out but bear in mind mutual inductance. If you place vias which carry current flowing in the same direction too close to each other there will be mutual inductance between them increasing the total inductance of the vias.  E.g. possibly making a 4×4 grid of vias look like a 2×2 or 1×2 at decoupling capacitor frequencies.  A good rule of thumb is to keep these vias at least one board thickness from each other or at least twice the distance between the planes the vias are connecting (more math).