Is it correct that one box of 17mm byou tacks have only 100 in a box? How come the square shank 18mm has 600 per box for the same price?

Yes. It's correct and one box of Nagado Byou Tacks (Round) includes only 100pcs of tack. One box is the minimum order quantity. So, we sell round byou tack from 100pcs and square byou tack from 600pcs.