you can create fireballs in a wide variety of types and sizes. You can generate them in sizes anywhere from a baseball to a meter in diameter, create them out of either burning gas, oil, or plant matter, and can control how they move, such as having them fly in a straight line, obey gravity, follow a set path, or even chase after specific targets. You can also set how they interact with others, such as having them disperse on impact, bounce off of people and/or the environment, explode and spread their contents over an area, or stick to whatever they contact.
There is no hard upper limit to the number of fireballs you can have generated at a time, but you can only generate about one maximum-sized fireball a second, or however many other, smaller fireballs that make up the total size.