Is the number of legs in myriapoda determined entirely by the genome?
Three factors that influence the number of legs are:

1) Sex : In some species of myriapoda, the females have been found to have more leg segments than males (reference) eg: Himantarium gabrielis 

2) age


  Growth is by adding segments and legs with successive molts
  (anamorphic), and myriapods continue to add additional segments and
  legs after they have reached sexual maturity (reference).


3) Working of a "segmentation clock" (set of genes) that periodically adds segments (reference).