Converting an 8″ round pan recipe to a tray of 12 cupcakes
Batter scales with volume, not with width. An 8″ round pan holds about 6 cups filled to the brim, and a tray of 12 cupcakes holds 6 cups — near enough the same that you can bake the recipe as written.
The number people miss is depth. Two pans can hold the same volume and still bake completely differently, because what matters is how thick the batter sits: volume divided by bake surface. Here that moves from about 1.1″ to 0.7″. Fill each liner about ⅔ full. Bake cupcakes 18–22 minutes at the recipe temperature; test at 18.
Cupcakes are the one conversion where time changes more than anything else: they're shallow, so they bake in 18–22 minutes rather than the 30–40 a layer takes. Fill each liner about two-thirds and don't be tempted to go higher — the overflow is what gives you those flat, spreading tops.
How many pans one batch fills
One 8″ round recipe fills about one cupcakes at the usual two-thirds level — a clean split, which makes this an easy conversion to do without scaling anything.
A caution worth reading
Volume conversions hold well for cakes, brownies and quick breads — anything poured. They hold less well for recipes where structure comes from the pan itself: a delicate chiffon or angel food climbs the ungreased side of a tube pan, and a loaf's shape is part of how it bakes. For those, scale the ingredients by all means, but expect the texture to shift, and use the skewer rather than the recipe's clock.