Basically, the licensing for software is per computer. According to v9 EULA, you will need to purchase a license for each computer at work plus one more for your personal computers. That means three in your case. However, we added some section to v10 EULA, and it allows you to use a second computer at home with the same license at work. Therefore, you need only two licenses total if you use v10. Not only because of license issue, I would strongly recommend v10 RC for its reliability and features, and then you can update to v10 official release at any time before July 31.
64-bit edition is new, and not all plug-in authors have 64-bit computers in the past. All my recent plug-ins are built both in 32-bit and in 64-bit. If plug-ins are the issue for you, you can always install 32-bit edition on your 64-bit machines.