Sold in a 4 pack, enough spacers for 4 wheels.  You need one spacer for each wheel on your skates.

These are ideal for wheels from brands like Undercover, Street Artist, Gawds, Dead.

Those wheels have smaller cores and will only accept the 8mm "slim" bearing spacers.

Spacers are found in-between the two bearings in the centre of your wheel.  Without spacers, your wheels won't roll properly.

Most inline skates use 8mm bearing spacers.

These will not work with K2 and other brands that use 6mm axles.