There are normally two to three layers (if fiberglass) in the outer skin but you'll need more than that to bring the repair close to the surface since you are digging into the foam but this won't really come into play for this type of repair. The fiberglass/resin repair should come out close to the surface so you are not using much body filler. Body filler should not be more than 1/8" thick (I usually try to stay less than half of that if any at all).

It's not that critical but it is my theory that you start with the larger piece first for two reasons:

1) The connection between the foam and the fiberglass skin is the important link. Starting with a large piece ties in the freshly exposed foam to that skin and makes a more unitized repair. After the first few layers of your repair, the fiberglass is just filling the indentation and isn't make as much structural connection.

2) If you start with small pieces, it's harder to guarantee full contact with the foam throughout the radius of the repair Lower layers of fiberglass may hold outer layers of fiberglass proud of the foam and leave a void around their enlarging perimeters.