Unicellular organisms have zero or one nucleus. If they have two or more nucleus is multicellular
Hmm, is that true? Looks like there are unicellular organisms with multiple nuclei:
Valonia vesicles are able to hold several smaller bubbles attached. They differenciate from the larger bubble by a process similar to gemation and quickly replace it when is too old or damaged and it bursts. If a bubble equals a cell in function and looks, an organism able to have rhizoids and other smaller cells glued to its main cell, is clearly pluricellular.