Plenty of cows have extra teats but we ignore them. However, here is one case where they were useful! I noticed a huge white charbray cow in a remote part of our farthrest paddock with four big bung teats and a new bull calf. The calf had no hope of getting on any of the main teats but the cow had two extras about an inch long right up the back of her udder. Calfie was sucking on those and getting just enough milk to stay alive, no more. (Those teats are plumbed in to the milk supply!) We went reluctantly down to grab the calf a couple of days later and bring him home (no mean feat, 700kgs of cranky cow to contend with). To my delight, I found that he'd got onto all the four big teats - over the course of the week, those teats had gone down a bit and softened up some from not being sucked. He was smart enough to keep trying them and had got on! Phew, thank goodness for those extra ones keeping him alive for the week.
Of course, had the cow been closer to facilities, we wouldve put her in the crush, immobilized her with the RAU, and put him on.
I saw her recently, those teats are small and neat now, you wouldnt believe they'd been so huge. We'll cull her at weaning time if we can.