Can I use this plugin on more then one site?
Yes, you can use S3Media Stream on any of your own sites. However, we give no support for your friend's of client's sites.
Web developers can get a special discount. Contact us in that case.
As for the JW Player license, there is at this time of writing no limit on the amount of sites but we don't control this and rules may change. Therefore, check www.jwplayer.com/pricing to make sure.