Why doesn't my solar water heater provide enough hot water for a few showers?
If it is properly installed and the auxiliary heater is working, you should have enough hot water for three or four hot showers.  A 200 liter tank is approximately 52 US Gal which is the normal size for a family of 3-4 people.
Adding a circulation pump will not resolve the issue you are having.  The circulation pump will shorten the time it takes for hot water to reach where your shower is located.  It will provided a small amount of addition storage, but since your tank is ~52 gal, the additional storage in the pipes will be minimum. 
Before adding a circulation pump, I would validate that the solar heater is working properly.  At a daytime high temp of 24-25C degrees, the water is still cold and needs the electric element to bring the temp to about 52C (125 F) in order to feel like hot water is coming out of the shower.
I would check that the thermostat is set to at least 60C (140F), the factory setting.  About 1 or 2 hrs before sunset, I would turn on the hot water and measure the water temperature.
If the water temp is 60C, then you know the thermostat and electric element is working.
If the water temp is not 60C, then you could have these potential problems:

faulty thermostat
Electric element not working
Electric element is undersized


Do you have the standard 4000 Watt-230 v heater or did you select another heater option?