Being a boiler engineer and also an electrician, I can say there is no such a thing as electric combi boiler. But if you search Google for “electric combi boiler” you will get a lot of results. I am not even bothered to read any of them.
The smallest output of all combi gas boilers is 24kW/h, which gives 10L/min flow rate at 35 degree temperature rise, ie, from 5 degree inlet to 40 degree hot water output.
In practice, a shower will use less than 10L/min flow rate, so the hot water temperature is higher than 40 degree by 24kW/h. But a combi boiler is still just capable of supplying a single shower at any time.
In a domestic house, the max output of a single electrical appliance is about 10kW/h, less than half of a combi gas boiler. This 10kW/h limitation is by the electrical consumer unit and the size of cables. You can’t get around this limitation, unless you have a three phase electrical supply in a house.
There is no such a thing as electric combi boiler, or they are not real combi boilers.