There are always some white cells in the ejaculate. Up to 1 million per ml is thought to be normal although in the USA they often allow 10 million per ml – so you can see how inexact a measure it is.
High numbers of white cells in the ejaculate can be caused by several things including infection. Usually such infection has been present for some time and has settled in the prostate gland. Long courses of antibiotics are necessary to cure this, as well as general health measures.