Oh. It is supposed to be 17 edges. When I saw that, it didn't seem right. However it was just a mistake. If you count the number of edges in drawing "F", you'll see that its 17. So 8-17+11=2.

Also you made a mistake here:

"I think that 8+16-11 doesn't equal to 2
so that part is a bit confusing". It should be 8-16+11 because the formula is V-E+F or in this case 8-16+11.

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