I've given this advice so many times since I started coaching strategic planning teams in 1981. Let's say your team can achieve 10 good-sized strategic objectives in the next 12 months. What will happen if you give yourselves a "stretch goal" and try to do 20? In my experience - almost nothing will happen. Your team, which would find 10 good objectives challenging and productive, will be lucky to get halfway done with each of the 20. At the end of the year, you will have a typically poor showing in the execution department and your team will have just a little less faith in the strategic planning process as a whole.
A good rule of thumb for objective setting is to have no more objectives than you have effective team members. For many companies, this puts the limit somewhere in the 5-10 range. Most companies I've worked with in the past five years have done very well with six objectives.