If you are going to use magic in the Vegas world, it will take a little extra work and imagination. It can also add an extra element of fun and creativity to your game.
Each mage picks a realm that they will draw their spells from. A common theme that binds the spells together like fire, water, healing, etc.
When the mage wants to cast a spell, he decides on what the spell will look like and what the desired effects will be. The rest is up to the DM. I take into account the characters level, stat bonuses, the defender's stats if they apply and decide on the effects of the spell. One of the key factors I also use is how well the player describes the casting of the spell and what the effects will look like.
As you can see, this means there is no limit on the number of spells a character can cast. It's all a matter of imagination and creativity.
Casting the same spell will not always have the same results. Magic being an art doesn't always respond predictably. Sometimes the results will be overkill and sometimes the spell will fizzle. There is no way of knowing when either will happen. Spells gone out of control is one of the reasons mages are hunted down and lynched. Oh, did I forget to mention that most people are fearful of mages and prefer to deal with them by using a tall tree and a short rope?
Magic spells all have a casting cost involved with them. This is the amount of magic points it takes to cast the spell. When a mage uses up all of his magic points, they can no longer cast spells until they rest up.
The cost of a spell is denoted by a lower case "c" following the name of the spell. "Heal c5" would be a heal spell that cost 5 magic points to cast. The cost of spells is designated by the DM and vary from character to character even for the same spell. The variance depends mainly on the characters realm of magic. A character that devotes his entire study of magic to fire will have a lower casting cost for fireball than a mage that studies all the elements.