We just bought a cabin. It has a 100 amp service breaker, and a service
panel with 5 20 amp breakers.

I know on appliances, all I have to do to find out the amperage is RTFM, but
for lights and such, how do I calculate just how much wattage I can put on
any breaker circuit?

The lighting is inadequate. I will need more. I have a licensed
electrician friend who will come and wire everything, so it will be done
right and safe. I just need to get an idea how many lights we are talking
about so I can do some shopping. I don't want to max everything out and put
up a ton of lights. I just want to balance them, and not put so many that I
am approaching critical mass.

TIA

Steve