There is a slice 0 for each of the planes, of course, no matter how the data is stored. I didn't remember which is used, but I just tested this out, and the slice numbering is effectively "LAI" (order not important here). So sagittal slices start from leftmost at 0 and increase to right; axial slices have slice 0 in the viewer at the inferior, and the coronal slice is 0 at the most anterior slice. Get the storage "Voxel Indexes" by right-clicking on the coordinate part of the afni GUI menu or through Define Datamode, Misc, Voxel Coords.