I’d guess at 60. All styles from thongs to shorts to waist high Sloggies, but mostly high leg. I’ve only a couple of “best” sets, but I have bras in white, black, pink, navy, red, green and cream and I like the colours to match.
As for why so many? Well they’ve just built up over the years and I don’t see any point in getting rid of any - it’s not as though anyone else would want them! They don’t take up much room. All my underwear’s in a long storage box tucked away under the bed, arranged in individual shoeboxes by colour.