To change the number of partitions in FM you have to set zen.lda.numPartitions. It's strange FM is controlled by LDA configuration. Perhaps this config property should be renamed to not include LDA in the name or a second configuration property should be introduced for FM?
FM.initializeDataSet calls DBHPartitioner.partitionByDBH which references LDADefines.cs_numPartitions which is defined as zen.lda.numPartitions.