There is no single normal number
Body size, temperature, workload, lactation, feed moisture and salt intake all change demand.
Measure when possible
Buckets, trough meters or marked containers can help you notice a change before dehydration becomes obvious.
Winter still matters
Some horses drink less when water is very cold. Heated or tempered water can improve intake where safe and practical.
Sweat changes the equation
Hard work in heat increases both water and electrolyte losses. Replacement plans should match actual workload and veterinary/nutrition guidance.
Sudden changes deserve attention
A sharp increase or decrease can signal management problems or illness and may warrant veterinary evaluation.