There is no single figure that counts as good. Net profit margin varies enormously by sector, business model and stage of growth, so a margin that would be excellent in high-volume food retail could signal a serious problem in software or specialist consultancy. The useful question is how your margin compares to your own history and to genuinely comparable businesses.
The main drivers of variation are how much value you add, how much competition there is on price, how capital-intensive the operation is, and how much of your cost base is fixed. Businesses that move large volumes at thin margins can be perfectly healthy; businesses with low volumes need much fatter margins to cover the same overheads.
Rather than chasing a target percentage, watch three things: whether your margin is stable or drifting, whether any movement comes from pricing or from cost inflation, and whether the margin converts into cash. Profit that never turns into money in the bank is a warning sign regardless of the percentage.
Any benchmark you find published should be treated with care — sector definitions differ, sample sizes vary, and accounting policies are not consistent between businesses. Your accountant can tell you which comparisons are meaningful for your accounts. This is general information only, not accounting advice.