How to use "off the top of one's head"?
"Off the top of my head I think I have raised 7 pets." Is it correct?

Question

Answer

off the top of (one's) head (adjectival)
In an impromptu way:
impromptu = 即兴

I don't know off the top of my head. = I can't think of it right now.

Answer

Yes. This idiom means to speak from recollection or to recall something without preparation.
Examples:
     1. He didn't prepare for the meeting,  he just spoke off the top of his  head.
  
    2.  I can't remember  all those dates off the top of my head, so  I'll need to check the documentation and get back to you.