I am always having a broken spoke once in every 500Km
Check if all spokes are properly tightened. if some spokes are too loose that can put extra stress on the rest of the spokes which can cause premature failure.
If so (some of the spokes are not properly tensioned) you should true the wheel (or have it done if you don't feel comfortable doing it yourself). 

Is the back wheel the original back wheel that came with the bike or has it been replaced with an aftermarket rear wheel? If so it might be of lesser quality (in which case the spokes will most likely also be of lesser quality). 

How old is the bicycle/how old are the wheels? Metal gets 'tired' after a certain number of load cycles ( https://en.wikipedia.org/wiki/Fatigue_(material) ).
(If the spokes break when trying to tighten them to a normal level the problem is most likely metal fatigue and you should either replace all spokes or replace the wheel (the latter is a more realistic option)). If the wheel is indeed quite old, the spokes are all properly tensioned and there is no other reason to suspect spoke failure metal fatigue might be the issue, if so (as afore mentioned) either replace all (old still remaining) spokes or replace the wheel (which is more conveneint/also cheaper in most cases). Spokes are quite cheap but rebuilding the wheel will take a considerable amount of time.

Where are the spokes failing (at what point on the spoke do they break, near the nipple, near the j-bent?) Can you add an image of the surface of the spoke at the break point? The type of surface at the failure point can give an indication of the failure mode (if it failed due to fatigue or overloading (putting too much stress on the spoke). 

I quote:


  Fatigue usually begins from a stress concentration at the surface. The fatigue cracks grow slowly and usually leaves a striated pattern that looks like a smooth sea shell. Then, when the crack has gone far enough, the object will break suddenly due to the stress in the small remaining area exceeding the ultimate strength. This sudden fracture will usually look different - rough or torn looking. 




Source: http://www.learneasy.info/MDME/MEMmods/MEM30007A/properties/Properties.html



Source: https://www.fastenal.com/en/3289/fastener-fatigue



Source: https://www.sciencedirect.com/topics/engineering/tensiles