Is there a point where any additional workers won't speed up the harvesting process of minerals or gas from a certain mineral field or refinery?
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Your third worker will gather at a reduced rate. Your fourth worker will not produce additional income.
Please note that MULEs can gather over workers, but not over other MULEs.
This results in a limit of 16 workers at full rate, an additional 8 workers at a reduced rate totaling in 24 workers per 8 mineral patches. Please note that this is a full page at the bottom of the screem, as each row is 8 units long. It is however advised that if you have less than 16 workers at an expansion that you transfer those of an expansion with more than 16 workers.
As for MULEs, you can have up to 8 MULEs per 8 mineral patches. The fastest income of a single base you can get is by 24 workers and 8 MULEs on a gold expansion.
For gas, it depends on the distance to the gas geyser. In some cases, full saturation is reached on your third worker, in other cases (diagonal placement) the full saturation is reached with a fourth worker. However, the difference between the third and fourth worker is minimal and probably only matters when you are at a high grand master level. And even then, there are probably other more important things to focus on. You don't need a graph for this, when you see a worker waiting outside the gas geyser for long enough you simply have reached saturation...
Here is a graph for gathering minerals with workers:
— Source: TeamLiquid Wiki - Mining Minerals