A Mathematical Argument Against Solipsism

A mathematical question arises from solipsism and that is, "Why is there only 1 being? Only I?".
Numbers make sense logically and I can count them up in my mind and I can count up other purely mental things like the number of thoughts I have or visualise dots in my mind and count them.
So I have proof that multiple things can exist; that 1 is not the only number.
(And if something can exist; if it has the potential to exist then it is likely that either it has existed; does exist or will exist.)
But there is one thing, if I am a solipsist, that seems not to conform to this rule of multiple things existing. There is only one thinking being (that I have proof of), me.
So does that mean that there can be no more than 1 thinking being? That seems to contradict what I know is true of other things like thoughts and dots.
It seems more logical, to me, to presume that thinking beings also conform to the rule of being multiple and not just 1.
Thus it seems more logical that there should be other thinking beings apart from myself.
Do I have any evidence of other thinking beings?
Well only those other thinking beings that I percieve through my senses?
They might be illusions ofcourse but their existence would conform to a rule that I know is provably true, that everything (apart from the non-numbers zero and infinity) can be multiple; that nothing is limited to just one.
My argument can be summed up in this way:
Premise 1: It appears to be a universal rule that all things can be multiple.  I know that all things (apart from zero and infinity) can be multiple. All the thoughts and dreams in my head are multiple and abstract things I imagine, like dots and lines, to count can be multiple.
Premise 2: If something is a universal rule then everything that exists should conform to that rule.
Premise 3: A thinking being is something that exists. I am proof of that.
Conclusion: There can be multiple thinking beings.
Furthermore, the existence of the many thinking beings that I percieve with my senses would conform to that conclusion. It makes sense that they would exist; I would expect them to exist because I know that multiple thinking beings can exist.