They should be fitted into a 10mm diameter hole to enable the T-nut to be knocked in. Always fit the T-Nut into the top of the wood rail and screw the leg in from below. Making shore the leg always sits flat against the furniture base.
The 8mm diameter dowel screw is the most common in use in the uk for legs and bun-feet.
Supplied individually so you can purchase the quantity you require.