One goal can have one, several, or many objectives. No, you can use whatever feels comfortable to you and your supervisor. Goals should be realistic, i.e., practical and achievable. Goals should motivate people to improve and to reach for attainable ends. For a goal to be motivational, the person must feel that the goal can be achieved.
Impossible goals demotivate and defeat the goal-setting process.
Likewise, easy goals do not motivate any more than unattainable goals. You should review your goals on a quarterly or semi-annual basis to check your progress and to make any necessary adjustments. Your self-evaluation should discuss your progress toward your goals and objectives during the past year. That progress, plus compliance with performance standards, should be the major criteria used in judging your performance. You and your supervisor should review your goals and objectives on a regular basis and revise as needed.