Is there a site where you can, I guess, "count" this stuff? Like how exactly would I know how many more grams of carbs would I need?
Well, the nutrition guide for P90X has this info in there for you. There are some quick calculations in there for each phase of the workout and it'll tell you all that info. Then you can use MyFitnessPal (or similar calorie counting app) to track what you eat.
I have to note, though, that the eating plan for P90X is mostly for weight loss. What are your specific goals? The first phase of P90X has a high protein/low carb/low fat eating plan, if I remember correctly. You might not want that if you're looking to gain size, but you won't lose any muscle while doing P90X
Here's a quick calculator I found on Google.
Looks like this will tell you which "level" to be eating at, which is in the guide. You can probably find this info online if you don't have access to the nutrition guide. Also, if you scroll down on that site, it has some info on each of the eating "phases". It's a pretty nice summary. Hope this helps.