Sorry!!
It is hard to say because the schedule changes week by week. There are blocks/modules (usually around 6-10 weeks long) where there are a lot of mandatories and some that hardly have any mandatories.
I'd say 98% of the time lectures are not mandatory. But you may have exams (obviously), labs, clinical trainings, small group sessions etc. that are. As I said, the frequency of these just depends. Next week is PBL week for us (I believe we do that twice a year from year 1 and year 2) and we have a mandatory everyday. But the following week we only have one mandatory for two hours. Overall, it is hard to get an average.
I do not like mandatory sessions - most of the time I feel like I would have gained more studying myself (except for clinical trainings perhaps).
And yes, the general consensus seems to be that anatomy at this school is tough.