KUKA.Ethernet RSI XML is free or not ?

no

it's cost around 1000Euro

My local Kuka told me it's 2650 euro.

the price vary (bundling). there is a difference between list price and discount price.
discounts vary (for system partners, is it purchased together with the robot or not etc.).
if it was an afterthought you pay more

I have a question:

if I have several robot need to install the software. should I buy one piece copy or several piece copies of the software?

you need a licence for each robot, but you will ask and get a discount for bulk licence