Let me give two simple scene of charged particles to make you understand my question. There is a fixed positive charged particle in the center.
Scenerio 1 : A fixed negative charged particle is introduced there at distance $x$ from the positive charged particle. Force of attraction between them is $F_1$.
Scenerio 2 : Two fixed negative charged particles (of the same charge as that in the last scenario) are introduced such that the positive charged particle is at same distance from them (i.e. $x$, same as last scenario). And the force of attraction now between the positive charged particle and a single negative charged particle is $F_2$. And the force of attraction between the positive charged particle and another negative charged particle is $F_3$
Now, is $F_1 = F_2 = F_3$ or $F_1 = F_2 + F_3$ or is it something else? I think it's answer is $F_1 = F_2 = F_3$ because the distances and the product of charges is same in the calculation of all three forces. I have taken all the charges as fixed in these scenarios so that there is no change of magnitude of force with time.
If the answer to the last question is $F_1 = F_2 = F_3$, then
We know that in an atom, for example, in a neutral oxygen atom there are 8 protons and 8 electrons, i.e., 8 positive charged particles and 8 negative charged particles. We know it's nucleus can only carry 8 electrons around it. Now my question is why can't it carry so many electrons, like why don't it carry 13 electrons or 14 or even 16?