Suppose we're playing the Dresden Files RPG, and one of the characters decides to go for broke. He wants to use fate points on 4 seperate aspects that are relevant, for a whopping +8 to the result. Is this allowed? Is there a limit to how many fate points you can spend in a round, and if so, what is it?
Dresden Files does not limit the fate points you can spend in a single roll. As long as you have justifiable aspects and enough fate points, you can go for it.
That being said, Diaspora, another good FATE implementation, does limit you to using one aspect from each scope, the scope being the source of the aspect, like your character, opponent, scene, system, environment etc. You may want to use that if you want a limit.