Section 7.6 Saving and comparing time machine runs says up to 6 runs. Does this mean I can delete a saved run and save a new one as long as total is always 6 or less, or I can only ever save 6 runs so I should chose very carefully?

The first interpretation is correct. If you already have 6 runs saved, it’ll ask which of those you want to delete before allowing you to save your last run.

Interpretation 2 would be cruel!