World of Warcraft advice...?

Hey, you're able to to have 50 characters per account and 10 per realm. To be honest, race doesn't really matter unless you take an aspect of the game really seriously. Each race has a very minor advantage such as a silence, a buff, stun ect. As you're new to the game, I don't think you'll really care about this. Just go with what you like the look of, but if you were to go with one of your choices, Undead would have a spell to get out of any crowd control and eat a humanoid, restoring health. Trolls are good for a damage increase buff and blood elves for the silence (stops target from casting for 2 seconds) and a restore in their energy. To conclude, they are all good choices and play what you like the look of.