This is a single 1/2" spacer that can be used to help mount your alternator. One possible use for this spacer is if you need to offset your adjustment arm to line up with the face of the alternator. Sold per item, this is a single spacer, not a pack of five.
This is one of a set of three spacers available, there is a 1/4" model (p/n 10-4048), a 1/2" model (p/n 10-4000) and a 1" model (p/n 10-4047)