Activating Q, W, and E (the "reagents") will each give you a different orby thing (or "instance") floating around you along with a passive effect. You can activate up to 3 instances at a time in any combination, so for example QQQ, QQW, QWE, etc.
His ult invokes the spell and gives you another ability based on the instances you have activated. Each combination has a different spell. At level 2 Invoke you can "store" a second spell as a 6th ability.
TL;RD
He has 10 abilities. He can use 2 at a time.
That means there are literally a hundred different ways to play Invoker.