Around 1 to 1.5%. The best thing to do is set one focus keyword and use it to about 1% density. But use a generous amount of LSI (supporting) keywords to reinforce the focus keyword. Anything over 2% density is bad.
There is no such thing as the ideal keyword density it is a myth as writing should be as natural as possible however you do need to establish a focus, of course, but whatever feels and flows right according to what you write is what is really right.