3-5 ish, plus the planning and evaluation. For most GFIs, there's not a set number of activities required, so perhaps think about how long they'd spend in total if they were doing it across 4 different sessions and try to give them a similar amount of time. Although if doing them all on one day, then you'll probably need to allocate time in a previous meeting for them to choose and plan activities so they know what they need to bring on the day as that's an important stage of the GFI.
Not necessarily. It might be one medium-size activity (say c30 minutes) or a couple of short activities (some of the quizzes can be done in 5 minutes) or could be half of a longer activity (making the clay candle holders in one session, and painting them in another).