var pageNumber = app.activeDocument.artboards.getActiveArtboardIndex() + 1;

// main loop

//loop through artboards

for(i = 0; i < app.activeDocument.artboards.length; i++){
    app.activeDocument.artboards.setActiveArtboardIndex(i);
    var pageNumber = app.activeDocument.artboards.getActiveArtboardIndex() + 1,
        pageNumberLayerItems = app.activeDocument.layers.getByName('page numbers').pageItems;
    //loop through text items
    for( i = 0; i < pageNumberLayerItems.length; i++) {
        if(pageNumberLayerItems.length > 1) {
            alert('only one page number per artboard to doofis. artboard '+app.activeDocument.artboards.getActiveArtboardIndex()+' has '+app.activeDocument.layers.getByName('page numbers').pageItems.length+' items');
            break;
        } else {
            app.activeDocument.textFrames[i].contents = "test";
        }

    }
}