Three numbers drive parlor sizing: herd size, target shift length, and turns per hour.
Turns per hour is the number of full batches your parlor cycles through every hour. Herringbones and parallels typically achieve 4–5 turns/h. Rotaries run continuously at 80% capacity — so a 50-stall rotary effectively milks 320 cows/hour.
Working formula Cows ÷ (shift length × turns/h × stalls/turn) = number of milkings per shift.
A 200-cow herd milked twice daily in a 2×8 herringbone (16 stalls) at 4.5 turns/h needs 200 ÷ (4.5 × 16) = 2.8 hours per milking. Achievable with one operator.
At 500 cows, the same parlor needs 6.9 hours per milking — your operator is in the parlor 14 hours a day. You've already lost the labour battle; jump to a parallel 2×16 or a small rotary.
Robot break-even Robotic milking (AMS) makes sense for 50–250 cow herds with high labour costs and a barn layout that permits 1 robot per 55–65 cows. Above ~300 cows the per-cow capital becomes harder to justify versus a rotary; below 50 the robot sits idle.