It's not about how many outlets, lights, appliances, etc. you have connected to the electric panel, it is how many of these things you will ever use at the *same* time!
Like one of those long long power strips on a shop workbench which may have 20 outlets along the length of the workbench. Well 15 or 20 amps may be plenty because you are only going to be using maybe one or two outlets at a time.
But if you had 20 different things plugged in and had them all turned on at the same time, then the circuit would not be able to handle the load.
Same thing with your house. You may be adding a room, but when in that room, you probably will not be using much electricity in other rooms. Or when other rooms, probably will not be using much electricity in basement.
My favorite "whole house" max amperage draw is thanksgiving. Lights on all over the house. Kids in bedrooms, adults in living room. Multiple burners/oven going on range. Dishwasher going. Maybe washer/dryer going. Microwave going. Refrigerator going. Maybe garbage disposal going. Relatives have RV plugged into outlet, etc.
Add up all the watts of everything you will ever use at the same time, then convert that to amps here...
(Under "Single Phase")
http://www.jobsite-generators.com/power_calculators.html