It is agreed to have one missile ABM system for each party. US chose not to have any because they had a two-type-missile defence, Spartan and Sprint.

Both missiles were part of the SENTINEL/SAFEGUARD systems. One site was allowed, not necessarily one missile type.

Russia didn’t, they chose to keep one type of ABM missile, and that only around their capital.

Russia’s system currently also has two missiles. Like SAFEGUARD, one is endoatmospheric, one is exoatmospheric. Granted, in the beginning there was only one type, GALOSH, but currently both GORGON and GAZELLE are in service.