1 unit of insulin to 10g carbs is a common starting point.
You may find this works perfectly (or near perfectly ... no injected insulin is as good as a healthy pancreas) for you or that you may need to tweak it.
As
@delmcp says, we are all different and some people find the ratio needs to be different for different times of the day.
Like so much with diabetes, the only way to tell is trial and error ... and lots of testing.
Good luck.