This has 8 bearings, most skateboard bearings only have 7
Skateboard Bearings are almost always size 608.
That's 8mm inner diameter, 22mm outer diameter and 7mm wide.
Rated by an ABEC number. ABEC stands for 'Annular Bearing Engineers' Committee' . The higher the number, the higher the manufacturing tolerances.
ABEC has been adopted by skate manufacturers, whereas industry tends to use the ISO Standard. ISO stands for 'International Organization for Standardization' or the DIN Standard 'German National Standards Organization' .
Higher tolerance bearings made of exotic metals or ceramics are expensive but are not necessarily the best choice for skateboarding.
This shows a comparison of the standards.
ABEC --------------ISO -----------------DIN
ABEC1--------------NORMAL------------P0
ABEC3--------------Class 6--------------P6
ABEC5--------------Class 5--------------P5
ABEC7--------------Class 4--------------P4
ABEC9--------------Class 2--------------P2