Work out server count for load instantly with clear inputs, formula shown and shareable results.
Capacity alone needs peak divided by per-server capacity at the target utilisation. Surviving the loss of one availability zone requires the remaining zones to carry the whole load, which multiplies the count by z/(z-1): 50 percent overhead across two zones, but only 33 percent across three and 25 percent across four. That is the main capacity argument for three zones over two.
Server count
base = ceil(peak / (per-server capacity x utilisation)); with zone loss = ceil(base x zones / (zones - 1)); per zone = ceil(total / zones)
Losing one of three zones removes a third of capacity, so the survivors need 50 percent more each. Losing one of two removes half, so survivors need to double.
Autoscaling handles growth but not instantaneous zone loss, since scaling takes minutes. The redundant capacity must already be running.