Humanure is great except for all the pharmaceuticals we put in our bodies and other junk that goes down the drain. A lot of the pharmaceuticals do not break down in our bodies nor in the soil. People dump a lot of random stuff into the sewer system. Why put that on the fields where we grow our food?
From a first principles perspective, would I be able to grow enough food for me from the waste fertiliser from me? my intuition is that there wouldn't be enough
There is a reason dairy farmers have to buy fertilizer.
Most dairy farms are very efficient at collecting all the waste the cows produce. They then empty the pits into a tanker truck and spray/inject it onto the fields. It goes a long way and helps complete the cycle, but your intuition is right, it alone is not enough.
Depends on many factors. If you are growing all your food on 1000 acres (only food for you, not selling any for cash) you can do this, and your decedents can continue until the earth/sun dies (assuming your family doesn't increase in size which is the normal thing to happen). If you are trying to do the same on 10 square feet you have a much more intensive operation and probably need fertilizer because not all processes are 100% efficient.
Some of the nitrogen in your waste won't make it back to the plants. There are other micro nutrients that plants need that won't make it back either.
Right now the norm is to combine wastes, add it to drinking water, and then spend a lot of money to get the water out of it again.