I may well confuse things here....
My IC Ratio (Insulin to Carbs) is 9g to 1 unit of insulin - so assume a slice of bread (they are all different as it depends on what they're made from and how big they are) is 22g of carbs.
Assume by a 2 sandwiches - thats 2 slices - so 4.8 units.
As
@Juicyj says - relative to time of day, activity, BG levels etc.
I'm on a pump - set up to do it automatically (called a Fully Closed Loop), so I configure it to watch for a BG rise, and hit the rise with insulin - I have that set so its reasonably aggressive to avoid highs, but not cause lows (which takes a lot of tweaking and still alters depending on weather, time of day etc) - sometimes its safer to declare the carbs.
The amount of insulin should be roughly the same even depending on the type of food - but something like a pizza I'd spread the insulin 60% up front, and 40% over 4 hours
For me carb counting is essential to make sure I'm within range as much as possible, though sometimes thats guess work (and my loop picks up the mis-counting when I get it wrong to keep me in range).