When we use <file > tag we can select only one file and upload it but how to select multiple files and upload them once on the server.
Not true, HTML5 and most modern browsers allow a boolean "multiple" attribute on the <input type="file"> tag that allows multiple files to be selected. This is the way I would go, or dynamically create new file inputs, if required, for older browsers.
Alternatively, there's SWFUpload. Honestly, I (naively no doubt) abhor flash and all things flash related so I've never tried it before, but it seems fairly straightforward.