1 - who can fill the witness details
You can fill the witness details online at your GST portal. You can provide details of any known person.
2 - Is there any bond to be created or just LUT is suffice for export
LUT is enough, requirement of bond is lifted long back.
3 - Is a single LUT is suffice for multiple exports during the year
LUT is for a financial year. You have to submit LUT every year for each financial year. One LUT in a year is enough for all the sales made during the year.
4 - Do we need to submit the LUT to customs
Every thing is online, so customs must be having details of GSTIN.
However in your export invoice you need to mention "Export without payment of IGST" and attach LUT application.