We have a 208 volts 400 amp ccicuit that we want to transform to a 480 volt cicuit with a step up transformer . How much more amps will we gain? Is there a formula for this ?

You will not gain amps, you will lose them.

What does your electrician say?

Yep-increase Voltage, decrease Amps.

I have two questions...
1. Why are you asking this in a forum dedicated to DIYers?
2. Why are you attempting something at a scale you do not understand?
The "transformer" you want to use will have all the specs included either on the unit or in the literature which will accompany it. It appears you are over your head in this one.