That depends on what you refer to as causes and effects. If you're talking about events as they're understood in everyday language, then yes, a single cause could have any number of effects.

If you're talking about entire system states, then there cannot be two different states simultaneously.

In an arithmetical sense, 3+5 results 8, for example, same as 1+7. I know it seems overly simplistic in this example but that is my intention, because it extrapolates to any possible extent. In this sense, is reality a cause or an effect? and, if two causes can result in the same effect, is "reality" objectively undetermined and determined only by our imagination, as how I imagined a subjective yet feasible cause?

In an arithmetical sense, 3+5 results 8, for example, same as 1+7. I know it seems overly simplistic in this example but that is my intention, because it extrapolates to any possible extent. — Marax

That probably depends on what interpretation of quantum mechanics you ascribe to. The Copenhagen interpretation would, I think, lead to the conclusion that a given cause can have one of a number of different effects, selected probabilistically. I am not sure it works the other way around.

Under any kind of hard determinism, on the other hand, all states are unique and follow from one another, so there couldn't be more than one path either to or from an event.

In this sense, is reality a cause or an effect? — Marax

I don't think the question makes sense, because the idea that there are causes and effects is one way to organise reality. Reality itself isn't in a causal relationship with anything else.

and, if two causes can result in the same effect, is "reality" objectively undetermined and determined only by our imagination, as how I imagined a subjective yet feasible cause? — Marax

Well, if reality is determined by our "imagination", it doesn't appear that we have control over said imagination. On the one hand, it seems logical that reality could only be some sort of imagination, since it must be some model in our minds, created by those minds. But reality is also really stubborn, reasserting the same principles again and again, at least on the macro scale.

Thank you. Now I have a clearer view of it, although I dont know if I will have more doubts about it when I finally figure it out on my own.
Regards.