On every button click you are creating a new instance of SWFUpload and then immediately trying to call selectFiles. But since the Flash Movie hasn't loaded it nothing happens.

You need to create you SWFupload instances during load. Then have your button call upload_no.selectFiles();

Another thing, you are passing a string into load_settings as upload_no. Then you overwrite the variable with a new instance of SWFupload. Each button passes in a different upload_no value. It looks like you meant to create several SWFupload instances but that is not what is happening.

Here is what I'd do:

var mySUInstances = {};
window.load = function () {
var settings = { /* all your different settings */};

mySUInstances["upload1"] = new SWFUpload(settings);
mySUInstances["upload1"] = new SWFUpload(settings);
mySUInstances["upload1"] = new SWFUpload(settings);
mySUInstances["upload1"] = new SWFUpload(settings);
};

function buttonClick(upload_no) {
mySUInstances[upload_no].selectFiles();

}

Since you never really know if SWFUpload is supported in the user's browser you should have some graceful degradation in your swfupload_loaded_handler. This can show the upload buttons (which start off hidden) after SWFUpload has successfully loaded.