It could potentially be up to 20 boards if your top ten percentage-wise are different than your top ten total posts-wise. Personally, I can view three more by hovering over the pie graphs on the left side for a total of 13.
Doing some quick profile math:
10 boards visible on the right, total 5,675 posts
3 more boards visible by hovering on the left, total 173 posts
Combined total 5,848 of 5,929, or 98.6%.
Sure, it would be interesting to know where my other 81 posts are - but I already know they're mostly in international and the various other regional boards, so while I would support this change, it's not a big deal for me and probably the same way for most other users.