Yesterday I wrote that you only need 13 sentences in your task 2 essay. For the task 1 report, I aim for 9 sentences:
- Introduction: 1 sentence
- Overview (summary): 2 sentences
- First 'details' paragraph: 3 sentences
- Second 'details' paragraph: 3 sentences
Remember that this is just my approach, and it's what I tell my students to aim for (some reports might contain 8, 10 or 11 sentences, which is fine).