7 Macs here. Two run Mountain Lion and have 8GB RAM. Activity monitor shows that's just fine even though I typically run a Windows virtual machine under Parallels, screen capture (iShowU HD), Keynote, Safari, and iMovie simultaneously. One runs Mountain Lion with 4 GB. An MBA that is basically only used for browsing runs ML with 2GB of RAM. A mini with Snow Leopard Server has 4 GB RAM. Two minis with Snow Leopard have 2GB of RAM and run Plex and occasionally Safari. None of these systems have memory problems.
So if you usage is light, 2GB is sufficient, even for Mountain Lion.
It's best to check your memory usage (particularly page outs over time) to determine if you need more memory. More memory costs money, increases boot (particularly POST) and sleep times, and takes more disk space for the sleep image.