We use this idiom when giving information that we think we know, but might not be 100% sure about all the details. In other words, it's information we're giving from memory without the benefit of trying to actively concentrate or do research. Example: How many cases of Coke are there in the supply room? Off the top of my head, I'd say there are about six cases. Do you know how many people she invited to the party? Off the top of my head, I think she invited 50. When is the next research paper due? Just off the top of my head, I think it's due in March. Can you name all the presidents of the USA? Off the top of my head, I can only name about ten. Hope this helps.

e.g. Q. How many Chinese restaurants are there in your city? A. Well that's a tough question. Off the top of my head, there are probably about 6 or 7. There's one over there....etc..