Most parents would agree that toddlers need a lot of socks. How many pairs of socks a toddler needs depends on several factors, including how often they wear socks, how often they go through the laundry cycle, and whether or not they have any particular sock preferences.
Assuming that your toddler wears socks every day and you do a load of their laundry every other day, we would recommend having at least 10-12 pairs of socks on hand at all times. This will ensure that your toddler always has a clean pair of socks to wear and that you're not constantly doing loads of laundry just for their socks.
Of course, if your toddler is particularly picky about their socks or if they seem to go through them quickly (perhaps because they're always getting them dirty), you may need to have even more pairs on hand. In these cases, it's best to err on the side of caution and have 15-20 pairs available so that you don't run out.
Ultimately, the number of pairs of socks your toddler needs is up to you and will depend on your individual circumstances. But as a general rule of thumb, 10-12 pairs should be sufficient for most families.