I am installing a fabrication unit and want to install 8 nos. 400 amps. Electric welding transformers. How much KVA power should I get installed ?

Power formlua is:

KVA= Volts x 1.73 x Amps

Amps will be:

Amps = KVA divided by ( Volts x 1.73)

You did not provide all the information needed, so I cannot complete the calculation for you.

Once you arrive at the actual total KVA value, you then need to consider any applicable derating factors to size your service or transformers.