That depends - are you using it as an electrical insulating oil? In that case, most manufacturers specify a maximum around 30 ppm.
For lube blending, the maximum allowable amount of water in any base oil is determined by your process - how much can you tolerate before the end product is hazy? You'll only find this out experimentally, by making bench blends to see what level of water causes you a problem, which then sets your spec. Then you find a supplier with product that is lower than that level.