One easy way to accomplish this is combining what was posted in the original post into a single statement:
int numberOfRecords = dtFoo.Select("IsActive = 'Y'").Length;
Another way to accomplish this is using Linq methods:
int numberOfRecords = dtFoo.AsEnumerable().Where(x => x["IsActive"].ToString() == "Y").ToList().Count;
Note this requires including System.Linq
.