On WoP sites there are around 900 assignments for Hogwarts, and with the inclusion of PG classes on sites applicable, this increases to over a thousand in many cases. On WoP UK for example, there are 1150 assignments which can be completed. For reference, UK has Flying as Y1-3 and Astronomy as Y4-7, and 7 PG classes with 4 as 3 year courses and 3 as 4 year courses (910 Hogwarts assignments and 240 PG assignments in total).
However, the assignments submitted achievement only goes up to 500 submitted, before jumping straight to all assignments.
Therefore, I believe an achievement for 750 assignments (and 1000 assignments for applicable sites) would be a good way to motivate students to pursue the top achievement of all assignments submitted to all courses, which of course is well over a thousand assignments on most sites.
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