Enterprise edition is a bit about features (TDE, Paritioning, etc) and you'd have to check if you need any of those.

It's also about scale, but that's hard to judge. 3 users can bury a SQL Server with poorly written code, large data sets, and complex calculations. Simple web pages can allow a SQL Server to support 10,000 connections.

If you look at the hardware limits, and you can reference some data to get an idea of load, you might be able to guess. If you're testing, use developer edition (don't but SE or EE) and see what level of load you can handle on hardware. Then decide what to buy.