How often can you need a touch up or enhancement after laser eye surgery?

Depends on your eyes. The corrective effects of laser vision correction are usually permanent. Occasionally, there can be a small healing response or rarely a thinning of the cornea that causes its shape to change and the prescription to shift. As well, as one approaches the 50s and beyond, changes in the natural lens of the eye cause prescription changes despite how effective lasik was.
As needed. Most people don't end up needing any significant enhancements but the answer to your question is as much as needed. Hopefully that's never, but for those who do, usually once is all it takes.