Why this page won't give you a universal "cows per acre"
Because there isn't one. An acre of managed Kentucky fescue and an acre of western range differ by a factor of twenty in what they grow. Any site that answers "how many cows per acre" with one number is guessing with confidence. What is universal is the balance: (acres × season yield × utilization × (1 − reserve)) ÷ (animal weight × intake × days) = head the land can carry that season. Get the yield number from your county extension office or NRCS ecological site description — it's the one input worth a phone call.
Animal units, if you need to talk that language: an AU is conventionally a 1,000-lb cow (typically with a young calf). A 1,200-lb pair is about 1.2 AU; a 1,400-lb bull about 1.4; a 700-lb yearling about 0.7 — scale by weight and stage rather than memorizing tables.
Stock lighter than the math: the calculator's answer assumes average rainfall and even grazing. The 15% reserve default is the cheapest drought insurance you'll ever buy.