The problem is the integer division. It truncates, which is not what you want here. Try:
int totalrows = dt.Rows.Count;
int PAGEsize = 10;
int totalPageSize = (int)Math.Ceiling(totalrows * 1f / PAGEsize);
... or, if you want to do it with integer arithmetic only:
int totalrows = dt.Rows.Count;
int PAGEsize = 10;
int totalPageSize = (totalrows + PAGEsize - 1) / PAGEsize;