By the time a baby hedgehog leaves its mother's nest, it has about 3000 spines on its back. As it grows older and bigger, more spines are added to maintain a suitable density of them. Consequently an average adult hedgehog weighing about 1lb 5oz (600g) has 5000 or so spines. A very big animal, over twice that size, might have 7500. Some books suggest that hedgehogs may have 16000 spines, but this is an exaggerated figure, probably based on a faulty method of estimation. That many spines would be a heavy burden to carry around.
The author of the above quote states that their figures are correct as they cut the spines off dead hedgehogs and counted them all!!