Most proposals have at least two specific aims.There is a fine line between an ambitious project and one that has no hope of being completed in three years. Think really hard before adding a third aim!
There are two ways to portray your specific aims within the proposal. One is to identify the general hypothesis that you are testing. Then align specific questions that will address that hypothesis. In this case, a specific aim is built around a hypothesis. There are usually 2-4 questions that you will propose to address under each aim. At this point the Aim and Questions are just a brief synopsis of what you plan to do for the research project you want to perform.
Another approach is to write a general statement that describes the intent or direction of your research (aim). Then define intermediate specific steps (goals, objectives) that will be taken to attain the stated aim.