Hi,
as already mentioned, the answer to your question is different for every person.
When I started on insulin, my doctor gave me a starting ratio of 1 unit of insulin for 15 g of carbs and I then adjusted from there. Adjustment was then done in combination with close monitoring of blood sugars. Ie. I test blood sugar before eating, lets say it is 100. I then eat 45 g of carbs and inject 3 units of insulin before the meal. Test 2 hours after starting to eat and my blood sugar is 140 (my target is < 120 after eating). This tells me that the insulin was not enough. Next time I eat 45 g of carbs I will try 4 units of insulin). I actually finally worked out that I need 1 unit of insulin for about 10 g of carbs.
It also helps to restrict carbs. I typically eat few carbs - mainly vegetables - and only need to inject 1 or 2 units of insulin per meal.
Ideally your doctor would refer you a diabetes educator who can help you with this.
The following books are also good references "Using Insulin" by John Walsh, or "Think like a Pancreas', both of which explain how to work out carb:Insulin dosing.
That is different for every person. Are you a T1 or a T2
you need to see the MD for that answer (I/C ratio)