Is 8GB of RAM enough for speadsheets, PowerPoint, and 20+ browser tabs?

It's a bare minimum but IMO not enough to future proof it. New OSs and apps require more RAM with every upgrade.

For a Mac with an SSD, as the MBA has, 8GB is sufficient for today. With 20 tabs open and MS Office apps, you'll certainly be bumping into virtual memory territory. That's where the SSD is nice to have. So today you'd be okay. Next year if you want to upgrade to the new Mac OS? Maybe, maybe not. Two years from now? Probably not. If you intend to use this computer for several years as your primary computer, in my opinion, you should go to 16GB. That's where I've gone with my two recent computer purchases.