What do I say about why I left my last job? after 10 years as a bookkeeper

They always ask why I left, and I freeze. I have been trying to stay organised about it. I have been practising in the mirror. Do you mention numbers?

Say it plainly and move on to what you can do. Interviewers usually move on if you do.

If a company has been in the news for layoffs, you can mention that. Often the interviewer already knows.