How many standards per term/unit/whatever is too many? ...too few? I've landed at around 25 per term (trimester) this year. For AP, I go with about 5 per unit, with 6 or 7 units total. The AP standards stay the same throughout the year, since the year leads up to the AP exam.
Too many standards means that it's difficult to assess them all enough to show any sort of progress during the term and/or year.
Too few risks having not enough resolution to pinpoint student difficulties. This makes it hard for students to identify where they need work, and for me to identify where to reassess an individual and how to steer the course. If you start thinking about the extreme case of this problem - only 1 standard, called "physics" - then you see just how poorly a "regular" grade communicates!