Mammoth Uncensored Word List:
15 words ending with ns, 21 letters long

adrenocorticotrophins anthropomorphizations compartmentalisations compartmentalizations counterdemonstrations duoquinquagintillions institutionalizations internationalizations microminiaturizations nonillionsedecillions photophosphorylations pseudosophistications quattuorvigintillions quinquatrigintillions sesquinquagintillions