Off the top of my head means answering from memory, without checking. Off the top of my head, about forty, I could not say off the top of my head.
It is a warning that the answer may be wrong, which is why careful people add it. It sits at the front or the end of the sentence and never in the middle.
1phraseoff the top of my headAnswering from memory, without checking.
“Off the top of my head there were about sixty of them.”
off the top of my head is a phrase.
Also accepted in games: off the top of your head
Practised in Idioms