Chromium(VI) phosphate has the chemical formula Cr(PO4)2 because it contains one chromium ion bonded to two phosphate ions. The phosphate ions each have a charge of negative 3, while the chromium ion has a charge of positive 6.
For chromium and other transition metals, a Roman numeral refers to the charge of the metal ion. This is necessary because most transition metals form ions of multiple charges. For instance, the (VI) in chromium(VI) phosphate clarifies that the chromium ions in this compound have a charge of positive 6. Since phosphate always has a charge of negative 3, two phosphate ions and one chromium ion create a neutral compound.