Electrochemistry conditions for voltage to increase in a galvanic cell I understand that increasing the concentration of reactants in a galvanic cell generally causes the voltage of the cell to increase. But is that really the case always?

Consider a standard cell of zinc and copper, 1 M and 1 L each solution of copper nitrate and zinc nitrate in each half cell. Another cell, increased the concentration of copper nitrate to 4 M, but added 0.25 L of it and kept the same zinc nitrate 1 M and 1 L.

This way I increased the concentration, but the amount of substance is the same. Does the voltage increase in the new cell? I would really appreciate someone to explain to me the specifics of what causes the voltage of a cell to increase based on the reactants
Yes, your voltage will change in accordance with the Nernst equation

$$\Delta E_\text{cell} = - \frac{RT}{zF}\ln Q_{\text{rxn}}$$

So, in your copper-zinc battery, where

$$\ce{Cu^2+(aq) + Zn(s) -&gt; Cu(s) + Zn^2+(aq)}$$

with a standard $E^\circ_\text{cell} \approx \pu{1.10 V},$ setting the $\left[\ce{Cu^2+}\right] = \pu{4 M}$ should raise your cell potential by

$$\Delta E_\text{cell} = -\frac{RT}{2F}\ln\frac{1}{4} = \pu{+0.0178 V}$$

for a new cell potential (at $\pu{25 °C})$ of

$$E_{\text{cell}} = \pu{1.10 V} + \pu{0.0178 V} \approx \pu{1.12 V}$$