What is the percentage of lanthanoid metal in mischmetall?

95%

Mischmetal is an alloy of rare earth elements. A typical composition includes approximately 50% cerium and 25% lanthanum, with small amounts of neodymium and praseodymium. 

It contains approximately 50 percent cerium and 45 percent lanthanum so the total percentage of Lanthanides is 95%, whereas the other iron is found to be 5%.