The free plan takes ten singles, and ten is enough to see the reasons and decide whether you believe them. What it is not enough for is your actual drawer.
A real pile is fifteen to thirty. What a pile yields climbs faster than the pile grows, because every sock you add can pair with every sock already there: 10 singles might give you three groups, 25 might give you nine. Uploading half your pile is not half an answer — it is a worse one, because the pairs you never see are the ones that needed the sock you left out.
The paid plans lift the limit, add the near-miss list with the exact gap, remove the watermark and print a sheet you can sort against. Two or three pairs rescued covers the year.
The watermark on the free plan never sits over the list of what the photograph cannot tell you. That warning is the same on every plan, because it is the part that stops you making a mistake.