if you have 4 / 5 / 6 batterys , and constantly change them on your spark , does anyone know if the spark gets too hot ?
any expierence ? or has Dji specified ?

I can only speak for my Mavic and Phantom (I don't have enough batteries for my spark yet) but I was never able to overheat one of them.
The Mavic cools down with the fan like the Spark. So you can fly as long as you want. Additionally I didn't read any warning from dji to let the aircraft cool down after a flight before you start again with a new battery.

10 minutes is long enough to fry electronics. If your Spark has issue, just 1st battery you will know. But Spark is designed to work as long as you have battery to run.

It doesn't hurt to fly consecutive battery's, it's not uncommon for lipo battery's to hit 145 degree Fahrenheit.