How many KM to change accessory belts + Water pump ?

Basically, if your car's engine is equipped with timing belt (not timing chain), then both timing belt and accessories belt should be replaced at the same time for peace of mind, as does the water pump.

You should either follow the car's manufacturer suggested changing time-frame, or do a visual test on the accessories belts and see if there's any cracks, lines, fractures, whatever it is.

Another reason to change all three all at once is to save money on labor cost. To change the water pump and timing belt you have to take off the accessories belt anyway, so it's cheaper to change all three all at once than to change one by one and pay the labor cost for taking off and putting back in the accessories belt each time.

it depends on the brand as well.

Usually japan car ~100,000 km
For European car ~80,000km

100.000km change time-belt and water pump.