Yes?
OP: 8 Demigods is FINE. IF there are more than 2 viable (competant) builds for a Demigod, and if those builds change the gameplay of the given Demigod, this is going to be just fine. It's just the start, anyway. More will be incoming.
Remember that, the game's developers can think, too! You aren't the only one who thought that 8 might not be sufficient. Difference is, they can do something about it. We'll see in beta 2.
You are thinking too much DotA, where there is 1, rarely 2 builds (of major difference) for each hero, and the rest are similar, for most.
Also, ever noticed you are buying the same items for some heroes, because their gameplay is too similar? 91 distinct heroes is not an achievement, it's impossible. Especially when the engine shapes the gameplay, instead of the gameplay shaping the engine. Some are bound to play like others.
I'm not saying all heroes only have 1, maximum 2 different builds, but those who have more, are mostly exceptions (for example give me Alchemist and I can make him pure pwnage DPS, pure tank, or pure support, just by different builds)
What I'm trying to say: They wouldn't put 8 heroes on a 5v5 game if they didn't know what they were doing.