I'm finishing a section of my basement, which currently only has one bulb for bare light. In planning the electrical arrangement of things, how do I estimate the load of light fixtures or electrical outlets so I know if I need heavier gauge wire, or additional circuits?

Yes, I could read the labels of the light fixtures and appliances I plan to install/use. But that doesn't allow for potential fixture changes or room re-utilization in the future. I'm thinking there must be a rule of thumb for "lights fixtures are this many watts, outlets are that many watts." I've just no idea how many.

**EDIT - Follow up question**

I'm seeing a lot of commentary which seems to imply 15 amp circuits are the norm (including other questions on diy.se). I know there are a few 15 amp circuits in my house, but most of them are 20. Why would you prefer one size circuit over the other? When would you use a 20 amp circuit?