Excellent question! The answer is actually a really cool and crucially important detail about membrane potentials and the movement of ions. Basically, the electric force is very strong. You don't actually need to move many ions for the membrane potential to fluctuate dramatically. The number of ions that "move" to establish equilibrium is tiny compared to the actual concentrations. That's why the Nernst and GHK equations are so powerful: what really matters is the relative permeability of different ions and the gross concentration differences, not the changes in their concentration that occur when, for example, voltage-gated channels open during an action potential.

You can calculate just how many ions have to move, which depends on the size of the cell. Here is a nice example from the reference I have linked below:

> For a typical cell, 1 microcoulomb of charge (6 × 10^12 monovalent ions) per square centimeter of membrane, transferred from one side of the membrane to the other, changes the membrane potential by roughly 1 V. This means, for example, that in a spherical cell of diameter 10 μm, the number of K+ ions that have to flow out to alter the membrane potential by 100 mV is only about 1/100,000 of the total number of K+ ions in the cytosol.


Reference:

[Alberts, B., Johnson, A., Lewis, J., Raff, M., Roberts, K., & Walter, P. (2002). Ion channels and the electrical properties of membranes.](https://www.ncbi.nlm.nih.gov/books/NBK26910/)

(as a side note, movement of ions *does* matter over a longer time scale, which is why it is necessary for the Na+/K+ pump to constantly operate to keep the membrane potential steady, and some ions like Ca2+ are at such low concentrations normally that the influx of calcium can be substantial relative to the resting concentration, which is partly why calcium is so powerful as a second messenger and why calcium chelating components of the cell are important)