how to insert timer on keynote slide?
I am preparing for a presentation and I want to insert timer on keynote.
Do you mean on your slides themselves?
And do you want one to time the slide? or the entire presentation?
There is no built-in support to show a timer on the slide. It is present (or, at least, available) on the Presenter's Screen, but not as part of the slide itself.
If you want a timer on your slides then you're going to have to craft that yourself. There are a few ways of doing it, from build in/out animations on objects, or a background video file that plays across your slides. Without knowing your goal it's hard to be more precise.