Choosing a batch size
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Limited by how fast you can pour and by where the bars will sit for six weeks.
What sets the ceiling
The working window between trace and unpourable, which for an accelerating fragrance is measured in seconds rather than minutes.
Mold capacity, and whether you can lift the container you are blending in.
And heat: a larger mass retains more heat, so a big batch gels more readily and overheats more easily.
What sets the floor
Lye weighing accuracy. Lye at a few hundred grams is easy; at twenty grams the tolerance of a kitchen scale is a meaningful fraction, and lye is the one ingredient where being out matters.
And fragrance, dosed at a few percent, which on a very small batch is a couple of grams.
A practical number
A batch of one to two kilos of oils for a hand operation: a loaf mold, ten to fifteen bars, weighable accurately and pourable inside almost any working window.
Scale by running more batches rather than larger ones until you have a stick blender and a mold system that make a bigger pour comfortable.
Batch size and cure space
The constraint people forget.
Every bar occupies a rack with air around it for four to six weeks. A month of production is a month of racks, all of it at once, none of it sellable.
Which makes cure space, not making capacity, the thing that decides how much soap a small operation can have in the pipeline, and it is the reason a soap business needs a room rather than a shelf.