Multiplicity only affects spells which can be stacked, and when a spell that stacks is affected by multiplicity, it creates maximum stacks at the cost of x times the spells cost (with other active metamagics added in as well.)
Niac's Biting Cold: 20SP. does 1d6+1 per caster level every 2 sec for 16 sec. Cast at level 20, it would do 1d6+20. Empowered, Intensified, Maximized would do 300% increased damage but the spell cost would be 75. If you also used the Multiplicity metamagic, it would cost 225 but start with a stack size of 3. BUT SO EXPENSIVE.
I guess the real use would be to just cast the base DoT with Multiplicity and then use the Empowered, Intensified, Maximized DoT afterward to refresh the stacks.