What's the usual IB contextual offer for Cambridge/??? and how important is it to maintain your score?
Cambridge doesn't do contextual offers. Their IB requirements vary by course (and sometimes by college).
As an example, Natural Sciences would typically require 40-42 points, with 776 at Higher Level. However, you should note that for this subject the majority of IB entrants achieve at least 43 points overall and/or grades 777 at Higher Level.