These bearing adapters are made of aluminum. The bearing adapter fits over your mini bearings so they can be used in a 608 wheel hub. There are two advantages to using mini bearings.
1. You create a much lighter setup as a 608 bearing weighs 12 grams compared to a mini bearing at 4 grams and an adapter at 1 gram.
2. Mini bearings spool up faster than regular 608 bearings. 

You need 1 adapter per bearing.