I have a big supply of slippery rayon yarn. An experienced weaver who has worked with this stuff in the past told me to glue the knots. I havent tried it yet, but I am going to.
At the risk of sounding harsh, there is no "rule of thumb" for determining how much mop cotton is needed for a placemat. First, "mop cotton" differs in weight, yardage and ply depending on the source. Second, placemats differ in size. The only way to determine how much yarn is needed for a placemat, or any project, is to learn how to do the math and calculate yarn requirements for your specific project and yarn.