Not necessarily. If you cut something, you're left with two pieces. If you cut again, depending on how you cut you can either have three pieces or four (think of a cake). If you cut 17 times, you could have a bucketload of pieces, or only 18, depending on how efficiently you cut. However, Shiki doesn't actually cut 17 times:
(both lines from s65)
Onna no karada ni aru kei jū nana ko no kuroi sen,
kanojo o, jū nana ko no nikuhen ni 'kaitai' shi ta.
The black lines that divide her into seventeen pieces.
I "dismantle" her into seventeen pieces of meat.
The first sentence gave my limited Japanese skills some trouble, but at least the second line seems pretty obvious as far as direct meaning is concerned. So if the translation is accurate, I must conclude that either the Mizuumi Wiki is wrong, or Melty Blood's Shiki uses a different attack on Arcueid than Tsukihime's Shiki.