Voltage follower output current
Let's say I am using an LM358 op amp to implement the buffer (unloaded), do I just use the Output Current given in the datasheet?

If the output is unloaded then the output current will be zero as there is nowhere for it to go. (It's an open-circuit.)

Or do I consider other factors here?

If there is a load then the output current will be determined by  that load. For a resistive load it's easy: \$ I = \frac V R \$. For a reactive load (inductive or capacitive) it gets a bit more complicated as the voltage and current will not be in phase.