Short answer: a 18 × 3 in river on a ¾ in board needs about 40 in³ = 22 fl oz (0.18 gal), about 25 fl oz with 10% extra. A 6 in, 2 in-deep resin bowl cast in a mold holds about 28 fl oz if solid (use the mold calculator), far less if you only coat it.
Some epoxies are rated food-safe only after full cure (usually 7+ days) for incidental contact. Don't cut directly on resin; finish the wood with food-safe oil.
A deep-pour epoxy for the river, since ¾ in is too thick for most tabletop resin in one pour.
See also: river table calculator · mold volume
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