Right now, there's just one rule: Do not call more than three names at a time. If you finish and submit a level, then you can claim another name. (This may be subject to change depending on how many people are working on levels.)
There's also the option of taking a name for a level that was actually used in MAGL X. (Or X2, when that gets released.) There won't be as many of these and they'll probably be bonus levels, so I'm going to say that people can only claim one of these at a time.
If I think of other rules, I'll note them here. In the meantime, just use this thread to claim names.
Level names and their claim statuses can be found on this spreadsheet here.
The original name list posts (only here for convenience in case one of us manages to horrifically bungle something on the spreadsheet):