Like owned said, it's all about simplicity. Let's be real here, there are a lot people who are noobs at Tibia. The majority of players will get overwhelmed if things are to complicated which will cause them to leave the server.
With that said it all depends on how your make your vocations. The thing I don't like about having so many vocations is that it's much harder to balance. You can go to any server you want that has more than 4 vocations and I can guarantee you that if you ask players they will tell you that X class is overpowered and or X class is under-powered. Even servers with 4 vocations are still unbalanced, and no one likes unbalanced servers. So honestly, it all just depends on how well made and thought through the vocations are.
My personal preference is 4 minimum or 6 max for vocations.
What I really liked was Ascalon's set up. You had 3 basic vocations (warrior, mage, archer) and then once you hit a certain level each of those branched into 2 vocations.
I felt like it was simple because all you had to think about was if you want to play a mage type class, an melee, or a ranged. You weren't forced to pick a specific vocation yet.
Then once you're playing you have time to think about what you want to become so you're not rushed into making a choice. You can ask other players who've already promote and see if it's something you'd like.
I personally don't like logging on and seeing like 10 different vocations to pick from because I'll be like "uh... um.. so many choices I have no idea what to play" and then I'll end up wasting a few days trying
to find the right vocation for me. I like it when you build up to something because that way you have an incentive, and something to look forward to.